# One family's solar story



## Sideways (28 Aug 2022)

This is intended to be a fairly detailed run through my own domestic solar PV project in the UK.


I'm a retired electrical engineer in my early 60's.
I'm as concerned as everyone else about rising fuel costs
We would like to be more "green". After travelling some 1.5 million miles in the course of my working life I've certainly done enough damage to the planet and agree that we need to consume less. This project wasn't done on the basis of financial returns.
Our house has a relatively large SE facing roof for the size of the place. Over the years we've felt how the sun can heat up the roof and have long felt that it would be well suited to solar energy.
We don't intend to move house - at my age I'm looking at this as a lifetime buy.
We were in a position to afford solar and any delay just meant we would get less benefit from the spend so now was the time to do it.


Four years ago I visited the "Solar & Storage Live" exhibition at Birmingham NEC.
This is an annual event mostly for the trade but open to the public.
It gave me an opportunity to see most of the major brands of inverters and panels in the flesh, to ask some questions and to identify some of the wholesalers who supply the installers and in some cases also the public.
Overall impressions: Solar PV was a mature technology and improvements are mostly incremental. Batteries are the new big thing. The industry seemed to be a bit down at the time. Feed in tariff had ended and demand for new systems was down.

I parked the idea for a couple of years and went back to the show last year, with my engineering head on, and specifically looking to shortlist the panels, inverters and battery that I might buy. I had no intention of doing the job myself but I would shortlist the kit and get quotes from installers who could supply and fit it.

From the show, solar panels that interested me :

Hanwha Q cells - Korea - PERC technology - very nicely built with good warranty
Jinko Tiger N - Chinese - N type silicon - good reliability scores
LG Neon R - Korean - N type silicon - good reliability scores
REC Alpha - Norwegian owned - N type silicon -
JA Solar and Trina - both Chinese - also had decent panels on offer but not as refined as the others above.

Inverter manufacturers :

Fronius - Austrian - premium brand, very high quality design and build
SMA - German - makers of the reputable Sunny Boy inverters - very well built
GivEnergy - UK - innovative and intelligent (as in automated) set of products but very new company
Solar Edge - German - well established and proven - broad set of products that work together and quite intelligent - have a very good solution for roofs subject to shadows - build quality looks much lower than Fronius or SMA to me

Batteries :

GivEnergy
There were a number of battery suppliers there including the majors like the Tesla Powerwall, but I found the products hard to assess and came away unconvinced. Few impressed me.

I was looking simply for high quality product, indication that the manufacturers were large and reputable, and for some degree of integration - not just a random collection of bits.
I was also interested in the solar panel mounting hardware because one of the real risks I anticipated was poor installation leaving me with a leaking roof and unsecure panels. I came away understanding that there are high quality roof mounts available and cheaper + less well made alternatives.

*Shortlisting the components

Panels*
Modern solar panels pretty much evolved from the space program decades ago. These panels use so called "P type" silicon. If you ever heard of a PNP and NPN transistors in school physics, it's the same idea. If not, don't worry and just follow along.
P type silicon proved to be the most long lived, and has been developed over the years. The latest flavour being called PERC. This is used to make the vast majority of panels and an excellent guarantee is 25 years plus a performance guarantee that the panel will still be achieving 85% of it's original output after 25 years in service.

A panel is simply a metal frame, an impact resistant glass top sheet covering typically 60 individual solar cells all connected by thin metal tracks that are printed, not actual wires, and a backing sheet of some type. A panel has a DC+ and DC- connection on the back and you wire several panels up in series (just like batteries in a toy) to make a "string".
There are some functional and cosmetic differences between panels - like the all black look and different ways of arranging the (printed) electrical connections that connect all the cells together in the panel. These can make small improvements in efficiency and make the wiring unnoticeable from a distance. The majority of modern panels are very similar in size and performance. Differences are subtle. The main thing is to buy from a reputable company that achieve consistently high reliability.

In recent years, studies have been done on the other type of solar cell. "N type" silicon. N type doesn't last as well in space because it suffers more damage from cosmic rays, but on the ground that doesn't matter and it is better than P type in other ways.
The upshot is that a small number of companies make premium panels out of N type silicon, and these can be bought with longer guarantees (25 years) and higher guaranteed outputs (90% of original output after 40 years !).
I decided that I wanted this technology. In a decade it will be mainstream but for now the choice is limited : REC Alpha (Norwegian), Jinko Tiger N (Chinese), LG Neon R (Korea) or Sunpower Maxeon 3 (USA + Mexico). The long life and high performance matched my lifetime buy objective and a price premium of 15% ish felt acceptable.

In practice, global supply chain was a nightmare this Easter. No one had any of the good stuff. I first shortlisted the Tiger N panels for value and while we were looking for them, I discovered that a lot of Chinese solar manufacture is tainted by reports of forced labour. Not OK.
I chose LG Neon next as this is a reputable Korean company, but LG withdrew from solar panel manufacture this summer !
Eventually my chosen installer stumbled on a supplier with Sunpower Maxeon 3 panels in stock, took 3 days to work up the courage, then suggested them to me. Maxeon 3 are the best made panels in the world. The company is Californian and the panels made in Mexico. I omitted them from my original list simply because I knew of their reputation and assumed they would be unaffordable. They cost me about 50% more than a good PERC panel but having realised how tough the supply chain problems were, I said yes. They have turned out to be the most impressive part of the system. Simply superb.








I'll explain the choice and layout of the panels later, but for now, I'll tell you that someone needs to consider the physical size and shape of the roof and work out the best way to arrange panels on it. I did all this for my roof and ended up with a simple, clean layout. 8 panels wide, 3 panels tall giving 24 panels in total. The maxeon 3 panels just fit my roof leaving enough space above and below. 375 W apiece x 24 = 9kW peak output.






*Inverter*
Solar panels output their energy in DC (direct current). 40 or 50Volts per panel is common. My Maxeon 3 panels make over 70V DC apiece. Panels are wired in series as "strings" that can be 400V, 600V, even more.
The inverter is a box of electronics that converts DC from the panels to 240V AC mains and feeds it to your consumenr unit through a circuit breaker - typically 20 or 30 Amp rated.
The AC from the inverter is exactly synchronised with the electricity coming in from the grid so that they play nicely together. It's just at a slightly higher voltage which allows the inverter to push it's energy into your home or back into the grid if you can't use it. The AC voltage from the inverter constantly adjusts, more sun, more power generated, slightly higher voltage to push that power out.

The inverter can be running 16 hours straight over a long day, handling several kWatts, and doing it 365 days a year for a decade or more. It's a pretty demanding job and the electrical eng in me thinks that a good quality, well designed and built piece of electronics matters if you want a long life and reliability. I was absolutely looking for something built to a "commercial" or "industrial" standard, not something that felt like a domestic appliance that you would expect to last 3 to 5 years.


*Batteries*
You have to consider batteries and inverter together.
When adding a battery to a solar PV system, there are two ways to do it.

First is to use an AC coupled battery which simply wires in to any new or existing system using 240V AC. The battery includes a charger. This approach is simple but has the advantage that an AC battery can be added to any solar setup, including ones already in service. Also, any battery can be matched with any inverter so you have freedom to mix and match. Tesla Power Wall is an AC coupled battery.

Second is to use a DC coupled battery matched with a specific inverter.
Batteries store power as DC. Solar panels make DC.
For maximum efficiency and maximum intelligence, the inverter can double up as a battery charger. It takes DC power from the panels and feeds it still as DC to charge the battery. There is no need to convert to AC in between and this improves efficiency. The battery pack doesn't have a charger built in because the inverter does this task.
The inverter draws DC power either from the solar panels and / or from the battery and converts this to AC to supply the needs of the house. This type of inverter is smart and has full visibility of the power in and out of the battery which a simple inverter with an AC battery does not. It is the better system if you are putting in a new, solar + battery system. The downside is that you need the battery and inverter to be designed to work together like this. That limits the available choices.

*Battery size*
How deep are your pockets ?
Our average daily electricity consumption is very typical for our size of home and family of 3-4.
Measured over the last couple of years it's 13kWh a day. We have gas hot water and central heating
The amount we use while the sun is down is maybe half that. We often use the oven for an evening meal.
We could choose a battery that held enough power to take us through the average "night" so just 6kWh say.
We could choose a battery big enough to carry us through an entire average day, so 13kWh
In the event, I decided that I wanted a battery that could feed a 6kW load - this is our oven plus a kettle in the evening or oven plus microwave. whatever.
It's a bit arbitrary, but small batteries can't supply that much power. and that 6kW load figure meant that I needed a battery with 16kWh of capacity. A bit more than one entire, average, day.
That's a lot of storage. About £10k and almost half the cost of the entire installation.
It means that we will be self sufficient - able to move the power generated by the solar to when we want it - almost 100% - for about 10 months of the year. Only in the two coldest months of the year is our solar generation predicted to fall to 8kWh a day. For the rest of the year, the solar will supply our average daily needs or much more and the battery will hold more than the average day's demand and deliver it when we need it. Overnight or during the next cloudy day.
If we have a run of bad weather we need the grid so never 100% but well up there.
Without the battery our self consumption will be hard pushed to reach 50%, 25-50% is more likely.
But ignoring inflation and Net Present Value calculations, I think the battery will take over 10 years to pay back and I genuinely don't know how long it will last for us. Potentially long enough and maybe much more, but this comes down to what we're willing to pay to be more green. We're willing to take the risk that this system will be more expensive than the grid in the end, in order to do our bit and not be so much at the mercy of governments and energy companies in future.

*Battery Chemistry and Life*
I didn't know as much about batteries when I chose mine as I do now, but reading around trends in EV battery tech is helpful.
For cars, people tend to want range and the greatest amount of energy storage for the least amount of weight. The batteries that fit that bill in 2021 and 2022 are Lithium Nickel, Manganese (?) Cobalt. So called NMC technology. This tech is expensive, uses Cobalt which is in short supply and a humanitarian car crash, more prone to fires, and lastly has a poor cycle life - around 1,000 charge discharge cycles before capacity drops to 80% of new. That could be just 3 years. Maybe 6yrs if you take it down to 60% of new capacity.
It's light weight but sitting in your house, who cares ?
For houses, Lithium, Iron (aka Fe), Phosphate (aka PO4) is a better tech. LFP batteries are more stable, less fire risk, much longer lasting (4,000 maybe 6,000 and max perhaps 8,000 charge discharge cycles) and cheaper. It's far heavier but improving. LFP batteries are used in buses and will be going into the base model Tesla3 made in China and many more new EVs. LFP batteries going in cars hold about 2x as much energy per Kg as the ones I'm still waiting for to put in the house. My battery stack weighs 1/4 ton, 250 odd Kg.

A huge factor in this is trying to guess what lifespan my storage battery will have. Cost divided by lifetime energy throughput will give a very real "pence per kWh" figure that has to be costed into all the energy that is temporarily stored in the battery.
LiFePO4 battery warranty is typically 10 years.
At one full cycle per day that's 3,650 cycles. A LFP battery should do that. An NMC battery is unlikely to be doing well after that many cycles.
LFP in fact may well be usable even at 6,000 cycles. At once per day, that's nearing 20 years as long as your battery was a bit oversized to start with.
20 years is somewhere near the calendar age limit of lithium batteries. They die of old age even if never cycled at all.
Putting all this into a hat and shaking it up, I figured my own eventual selection of batteries have a chance of lasting closer to 20 yrs than 10, and will cost me somewhere near 12p / kWh stored over that lifetime. A long way from free, but a lot less than the power company charge if I can't store my daytime generation and have to buy electricity in the evenings.

*Inverter and Battery Selection*
In the end, I had only two serious candidates because I wanted a "hybrid DC" inverter and battery solution described above.
One was "GivEnergy" - a very new company based in Stoke on Trent, UK but with overseas investors and partners. They are growing very fast because they offer inverters, batteries and an intelligent control that works with some of the most innovative energy company agile tariffs. The snag is that their inverter is 5kW and battery 8kWh. I would need two inverters and two batteries in my system and even then it was only rated for 5kW maximum discharge from the pair of batteries. It's a new company, unproven.

The second, and the option that I chose, was to pair inverters made by Fronius in Austria with batteries made by BYD in China.
These are separate companies, but they are cooperating to make their products work together to deliver a solution. BYD batteries are tested to work with Fronius inverters.
Fronius are a family owned firm that specialise in power electronics. They make just 3 groups of product: welders, forklift truck battery chargers and solar inverters. The core competence in power electronics is the same for all three.
They have an excellent reputation and I own one of their Magicwave AC DC Tig welders. It's design and build is very, very good. Up there with Kempii and Miller, arguably better than both. I've also dealt with their technical support and found them good. All this gave me confidence that the inverters should be good.
BYD was a real unknown, but if a respected supplier chose to partner with them they might be OK. It turns out BYD are the largest manufacturer of batteries for industry / traction application in China and I think the world. They make more batteries than Tesla and their batteries are the ones that power the new electric bus fleet in Sydney, Aus. Their batteries are LiFePO4 type prismatic cells that are better suited to long life needs of domestic energy storage than the car battery technology used in the Tesla Power Wall. Only time will tell if this is right, but an Australian facility that has been testing solar storage batteries and making the results public has tested the BYD battery and it lost very little capacity. The results give me some hope that battery could manage a 20 year useful life, maybe more,


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## Sideways (28 Aug 2022)

*Panel layout, strings and generation forecast*

Here are some things that I discovered about panels.

Firstly, they can be installed either way, "portrait" orientation or "landscape".
Initially, as our roof is rather wide compared to it's height, I made all my layouts with the panels in "landscape". Later after chatting with my installer, I accepted their preference to install "portrait" style and reworked the layouts.
The difference isn't anything dramatic. They simply find that it is easier to install the supporting roof brackets and the extruded aluminium rails that the panels clamp to when the panels are "portrait". If you need a landscape layout, they will need more brackets and rail and the installation will be more expensive.

DC wiring for solar panels isn't done using mains cable - or at least it shouldn't be. Beware of the cowboys. A special cable is needed
1. because it will be installed outdoors behind the panels and needs to be durable to cope with exposure, wet, heat and abrasion for decades.
2. because dc voltages are likely to be several hundred volts. Much more than the AC mains. So it is made with two layers of high performance insulation.
3. because the layouts often daisy chain a single conductor from panel to panel, not two cores and an earth
4. lastly, cable and special waterproof, locking, plugs and sockets are made to work together for a safe installation in the exposed conditions of the roof.
The wire looks like radio or TV coaxial cable but isn't. It has a very robust stranded wire in the centre to carry the power. The best quality plugs and sockets are by Staubli / Multicontact of Austria and these are crimped onto the DC cable using a special crimp tool (£100+ if you want to DIY the panel installation). My installer let me joint some at one point. It's not hard if you have someone there to demonstrate.

How many panels ...
There are some simple tools that allow you to predict how much power you will get from a roof. I'll link to them. They are available to all.
First you identify where in the country you are. UK has been split into numerous areas for this solar forecaster because hours of daylight, angle of sun etc are different around the country.
Second, you need to know the angle your roof slopes at from flat to vertical.
Third the direction it faces, like 6 o'clock / due south, or whatever.
Lastly, you enter the peak power of your array. X many panels x however many Watts per panel.
With these simple facts, you can look up standardised forecasts for how many kiloWatthours your installation will make in a year. Also, you can compile a bar chart graph showing how many kWh for each month Jan to December
This information is super useful, and all MCS (Microgeneration Certification Scheme) installers must use the same data and same methodology to make their forecasts. This lets you compare their forecasts with confidence.

My approach was very simple. If we were going to the bother of installing solar, make it as big as possible. The the cost of a 9kW array isn't 2.25x the cost of a 4kW one. It's less. The bigger we went the more chance of it making useful power in winter when it's at it's weakest.

Panels stacked in the garage waiting to be fitted.











*Angles and slopes*
Our roof stands at 50 up from horizontal. A little more than ideal for max power over the year because the sun will rise higher in the sky in summer when it generates most, and will hit the roof at a slant, not square on. It's actually better for the low sun in winter but that doesn't add as much to the annual total. Regardless, being a few degrees to steep or too flat doesn't make much difference, don't sweat the angles.
Our roof faces south east. Not perfect like due South but pretty good. Again, don't sweat it.
We can see the effect of south east. Our panels start to work at 6.30 am and do their best work in the morning. By 4.30 to 5pm the sun has gone around the end of the roof and generation plummets quickly from say 3kW to 1kW for a final 1-2 hours of our solar day.

A friend with East and West facing roofs put panels on both. He gets a long day of generation with each roof working well for half of the day and overlap in the middle. Don't let the angles put you off. Put your own numbers into the model and see what you can expect.

*My own forecasts.*
We use 3700 to 4500kWh a year according to our last 2 years bills.
The solar forecast said 9kW peak of panels on our roof should make nearly 7000kWh over the year.
The profile said 8kWh a day average in the two coldest months (we use 13kWh a day on average). Not self sufficient but not bad.
For 9 or 10 months, we should get at least our average daily requirement, and 30 odd kWh a day in the summer months. More than 2x what we need. Enough to make surplus for heating hot water and eventually, charging a car.

Since the system went live at start of April, we see daily outputs of 20 to 30kW a day, 700, 800kWh a month. A quarter of our annual demand in a single month if only we could move it to when we need it.

*Shadows and strings.*
You need to be aware of trees and buildings that can cast a shadow on your panels.
If even one panel is in shade, the power generated by the whole string of panels in series that it belongs to is reduced a lot. It is just llike having one weak battery among several good ones in your cordless drill. That one weak cell ruins the whole pack. In solar PV terms, just for as long as the shadow is there. It doesn't do any damage, it just means you don't get anywhere near full power.
So when your installer decides how to connect up the panels, he needs to look for shadows and may wire the panels to minimise the effect.
Just like battery packs can be wired in combinations of series and parallel, so can solar panels.
We have a neighbouring house parallel to ours that throws a shadow on our roof in the early morning. The shadow moves down the roof over 1/2 to 1 hour (suummer and winter) so the top row of panels sees the sun first, then the middle and finally the bottom row.
Because of the the way the shadow affects the whole width of the roof the same way, our panels were wired in 3 strings of 8. Top row, middle row, bottom row.

((Edit at Early November: the shadow moving down the roof is very real. At this end of the year the roof wakes up much later in the morning - 9.00 odd, and the lowest row of panels isn't in full sun until after 10.30. Our solar day has become much shorter. It can still generate very useful power but the real hit is more days of poor weather and consequently fewer hours of clear sunshine. I really need those batteries to capture the energy when we see it.))

Here are the two DC+ and DC- inputs of one of the inverters. The black outer / whire inner cables are the high voltage DC single core cables down from the panels.





The installer has to ( and in my case I did) check that the typical and the peak voltage of each string of panels, and the power that they make, match what the inverter is designed to accept. This is all about the electrical design. Interesting to nerds. No one else need worry about it because it's the installers responsibility to do all that. But it may mean an inverter isn't suitable for a certain combination of panels.

By this time, the inverters were wired in and a row of new AC and DC isolating switches installed so that everyrhing can be turned off for maintenance











Lastly. If shadows are a real issue for you, there are two brands of inverters that have a technological fix to overcome it.
Enphase brand micro inverters are small weatherproof mini inverters that fit on each solar panel. One inverter per panel means every panel can adapt to make it's own maximum output without affecting any other panel. This is a costly but high quality solution if you have difficult shadows on your roof from trees power lines and objects.
Solar Edge do something similar. They have a central inverter like normal plus a small box of electronics on each panel called an optimiser. It's a half and half between the standard design of one shared inverter and the Enphase solution using micro inverters on all the panels instead.
These two are both good, but more expensive and unnecessary for many people. My shadow problem was simple and short enough not to need this.


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## Sideways (28 Aug 2022)

*Inverter Details*

Once I had worked out how many panels I could fit on our roof and got a peak kWh number based on the N type panels I wanted ( they are very similar from brand to brand at any given physical size) I started looking at inverters.
I did get quotations for a GivEnergy system, but after the second go around when I realised I'd need two interters, two batteries and it was all going to cost over £20k, I focussed on the Fronius products. I may as well have top brand gear at that price point.
I found Fronius have recently introduced a new range of inverters called GEN24.
Their previous generation have been selling well for a decade or more. The new ones are hybrid DC inverters designed to work intelligently with batteries. Their industrial design is very nice from an engineers point of view.

The largest GEN24 inverter in the single phase range is 6kW.
I'd chosen a 9kW array. Hmmm.
But there is a principle in solar PV that connecting more panels than the inverter is rated for is actually a good thing. It's called oversizing. Everyone knows about it and the manufacturers test and certify their product as safe upto quite a high overload because of this.
The reason is that a) maximum power peaks don't happen often, and b) inverters need a certain minimum voltage from their panels to start working.
Oversizing the array means the panels reach the inverters minimum voltage quicker and drive the inverter better when the light is weak. It also means the inverter is occasionally overdriven in peak conditions. The inverter just "clips" - it adjusts so that the excess power isn't taken from the panels. Effectively it is wasted as heat. No harm done. The gains that oversizing the array gives in low light outweigh the loss of peak power. Oversizing by 20% to 50% is common. The Fronius inverter is safe upto about 70% oversize.

So I was set, and we ordered a 6kW GEN24 inverter to match the 9kW of panels. Both the installer and myself used Fronius' sizing software to check compatibility with the planned 24 N type panels and this recommended some wiring configurations for good efficiency.
One string of 8 panels would connect to the #2 input of the inverter.
Two strings of 8 panels would connect in parallel to the #1 input.

Not all inverters have two inputs but it is useful for a larger array. It allows panels to be grouped, for example an east facing and a west facing string can be connected to the same inverter. In mine, each of 3 rows of panels made a string, top two rows in parallel because they get the light first, bottom row that is the last to wake up would go into input 2.

When the inverter was delivered and installed I was impressed. Excellent quality.
It has built in wifi. You connect direct to the inverter to configure it from a phone app or a web browser - things like telling it what country it's in sets things like maximum voltages, what to do if the voltage goes too high, how quickly to shut down if anything misbehaves, etc.
The installation included it's own smart meter. Not a normal utility type, this is a Fronius part and part of the intelligent controls. With this, the inverter can be programmed to limit export to the grid to any value. So if the electricity company infrastructure can't accept your power, your inverter will deliberately reduce it's efficiency to limit export to whatever you agree with them.

Day to day, the inverter is linked to our home wifi and uploads data continually to the Fronius server which you can access via an account on their website to see all the graphs, manage users, do software updates, etc.
You can do the same with Fronius welders and the GEN24 is likewise straightforward and easy to control. Just complicated if you really want to get into it rather than leaving it to the installer.

I hit one snag shortly after committing this project. When we decided to buy the maxeon 3 panels rather than Jinko tigers, my installer ran the fronius sizing software with details of 24 maxeon panels. It turned out to be too much for the inverter. Just by one panel.
Rather than change the array design, we decided to fit 2 inverters. The 6kW one as planned plus a second 3kW unit from the older Primo range. This gave an exact match with no oversizing and we installed otherwise as planned. The 3kW inverter is fed from my #3 string. Strings 1 and 2 drive the 6kW GEN24. This is why you see two solar sources (2 inverters) on the myenergi app.
The batteries will be charged only by the 6kW inverter, but of course it can draw on the solar energy produced by the 3kW inverter as well as the power of the panels connected directly to it.





Here are the two inverters (newer design above) newly fitted before wiring in. With the cover removed, the innovative air cooling design and die cast aluminium case + heatsink of the GEN24 inverters is unmistakeable.

With 20:20 hindsight, I would choose both inverters to be new GEN24 models next time. The older design Primo is very reliable but not as smart. But that would be a few £hundred more expensive.

I also need to add as a postscript (Nov 2022) that just like Microsoft, Google and Apple, Fronius software updates are a long way from perfect. I am a reluctant updater when new softare releases come along and my scepticism has been proved right. 3 releases back Fronius screwed something up in their firmware that messed up my inverters and they haven't fixed it in the two updates since. The system still works but with multiple resets and error reports daily. This is an outstanding issue waiting to be fixed. If they don't, I'll be sending mine back and adding a strong DO NOT BUY to Fronius inverters of any model.


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## Sideways (28 Aug 2022)

*MyEnergi smart controls*

There is little argument that the way to get maximum value from solar PV is to maximise self consumption.
Key to this are
1. A solar diverter that measures surplus energy that you are trying to send back to the grid and instead sends exactly the surplus amount of power into an immersion heater or some other electric heater like electric radiators.
2. A smart electric vehicle charger that does the same for charging an electric car. It has the option to send only the surplus generation to charge the car battery.

There are plenty of EV chargers but few that are smart enough to only use surplus energy whenthat is what you want to do.
There are a few options for solar diverters.

The best system on the market at present is offered by a UK company called myenergi.
They have a long technical history in the game and offer both of the above, plus an app to visualise and control both.

We aspire to an EV so the solar was designed with a car in mind. Within days of installing the panels, we realised how much surplus we had and I went online and bought a Myenergi Eddi solar diverter immediately.

Eddi works great. It cost £300+ but it is well built and pumps several kWh a day into our immersion heater. Once the tank is hot and the immersion thermostat opens the circuit, Eddi realises this and sends any more surplus to a second circuit, which could be an electric rad or a second immersion heater.
This is actually smart. A big tank with immersions top and bottom can be set to heat water at the top of the tank first for a shower then preheat the bulk of the tank through the day.
While heating the second load, Eddi switches back to the primary load every 15 minutes in case it will take more.

Here is Eddi heating our immersion on priority 1. Note the periodic spikes happen when it checks back and manages to push a little more heat into the tank.




Also notice the dotted temperature line.
Eddi has an optional sensor circuit board. An end user installable thing. This allows you to control some extra circuits and to connect one or two standard temperature sensors. The dotted line shows the actual temp of our water measured by the sensor. You can allow that to control the heating.

Below is happens when we use hot water in the morning and and Eddi has time and surplus available to reheat the tank. On occasions we have heated and used three tank fulls of water in a day. It takes about 12kWh. In our old hot tank we are limited because the immersion only reaches halfway down into the tank, but at 70C this is enough for a good tub.





The system has upto three current sensors that can be hooked up to the meter tails to see what is coming in and out of the grid, and in my case to measure the generation from each of two inverters. The myenergi app shows all the energy flows in one place and is our goto in deciding whether we can run our kettle, oven, microwave, tumbler, etc for "free"

Here's the app showing the multiple devices. When we add a Zappi 2 car charger, there will be one more petal.





And some screens showing history and consumption in different ways
First shows 15kWh of generation. We used only 1.2kW of that and exported the rest.
We had to buy 5kWh from the grid because we don't have our batteries yet.
But the app doesn't count the energy diverted to hot water so we need to add about 8kWh to the 15 in the first screen, we actually generated about 24kWh and used nearly 10kWh of it.











This screen shows generation by the smaller our two inverters.
Notice how it starts at 7am. The small inverter is fed from the lowest our panels, the bottom our roof gets sun about 30 minutes after the top first gets it. So Eddi starts heating the water at 6.30 thanks to the larger inverter and the little 'un kicks in a bit later.

For both inverters the best power is made in the morning and early afternoon thanks to the SE facing direction of the roof. When the time comes, we'll be able to charge the car in the morning and go out in the afternoon


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## pe2dave (28 Aug 2022)

@Sideways , you say "First is to use an AC coupled battery which simply wires in to any new or existing system using 240V AC."
I don't understand that idea, explain please? 

Excellent and thorough view, much appreciated.


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## Iestynd (28 Aug 2022)

Love this. Very timely for us too as we are thinking about solar! 
Subscribed to this thread for future update. Thank you for taking the time to detail this out.


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## NormanB (28 Aug 2022)

Sideways said:


> This is intended to be a fairly detailed run through my own domestic solar PV project in the UK.
> 
> 
> I'm a retired electrical engineer in my early 60's.
> ...


Thank you for sharing your knowledge and experience in this thread.
A few questions if I may:
1. Whereabouts do you live (ish)?
2. Who did you use for sourcing and installation?
3. Did you consider ‘in roof’ as opposed to ‘on roof’ panels?
4. Did you consider Victron inverters/control gear?


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## Sideways (28 Aug 2022)

NormanB said:


> Thank you for sharing your knowledge and experience in this thread.
> A few questions if I may:
> 1. Whereabouts do you live (ish)?
> 2. Who did you use for sourcing and installation?
> ...


We live in Cheshire, UK
I was initially very happy with my installer but as time passed came close to starting legal action against them. The problem has been the BYD HVM high capacity batteries. These were promised for August against the initial installation completed eary April. Unfortunately supplier Midsummer Wholesale reneged on the battery order some weeks in and my supplier had to reorder from a different wholesaler. They didn't tell me this and ignored my email requests for update for a solid 6 weeks. Comms are now a little better but the HVM battery is still pending, latest forecast (as at mid Oct) is 2nd half of November.
I'm not quite ready to name and shame because the supply problems are not their fault but customer management has been incompetent verging on intolerable and I'm not entirely sure I won't have to drag them into court before I'm done.

I think "in roof" is great. It looks smart and would be the way to go with a new build or a reroof. Two caveats though. First is that mounting solar panels flush with the roof means little or no ventilation behind them. Solar panels get hot and generation is reduced when they are hot. So in roof is electrically less efficient because the panels run hotter. I haven't quantified this. Second is that some in roof systems use or require specific panels. The in roof systems on show at the trade show I mentioned all used panels that were just a mid range PERC technology. My preference for the high spec N type panels wasn't negotiable.
Victron equipment looks excellent to me. They have a very good reputation for off grid solutions and the equipment appears well made. I'd recommend anyone interested to consider them.


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## Lorenzl (28 Aug 2022)

A youtuber with lots of excellent DIY information - Will Prowse. He seems to be concentrating more on batteries and inverters at the moment and is based in the USA.


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## Ollie78 (28 Aug 2022)

This is very good information. Maybe I missed it but are you using lifepo4 batteries or lithium ion ?

Ollie


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## Sideways (28 Aug 2022)

LiFePO4 
They haven't landed yet but it will be a BYD HVM battery stack.
7 modules of 2.x kWh apiece making 16kWh
It looks like a stack of shoeboxes and stands about 1.6M tall x a couple of feet wide.


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## Sideways (28 Aug 2022)

*Everyone loves graphs.* *Here are our generation figures for the first few months in operation*:

You're interested in the top line. These are the sum of the 6kW inverter (purple) plus the 3kW inverter (green).
Apart from a few dull days where generation drops into single figures, we've mostly been above 15kWh and at peak, all the way up to 45kWh a day.

April
We switched on on 1st April, but had some teething problems that took a month to sort out.
It wasn't a big deal but for most of the month we were generating only about 2/3 of the system's capacity and for a few days I turned it off altogether.





May




June




July




August





And September. The generation is falling off. We had our worst day to date of just a couple of kWh in late Sept, followed by 20kWh the next day. That big battery would have helped even those out !






April through August monthly totals. About 3800kWh over 5 months.
That's our entire annual consumption if only we could move it to when we actually need it !
Then I've tacked on September's plot.


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## Sideways (28 Aug 2022)

2022 year end update

It got lost in the ether for a long time (i.e. installer didn't do it or follow up properly first time around so had to make a new application) but I'm pleased that our DNO agreed to the G99 application and we have permission to export upto the max 9kW our system is able to generate.

The battery saga has been an utter nightmare.

Midsummer Wholesale dropped my installer in the poo some unknown number of weeks after they placed the order for the HVM battery and announced they would be unable to fulfill the order. Along the way July had become June or September. The battery was reordered via another wholesaler but I wasn't notified of this. Deadlines came and went. Slipping a month each month through Sept, Oct, Nov and now the latest forecast is for the battery to reach the wholesaler on Dec 26th (yeah, right !).

I can't be bothered stressing about it so the installer has been warned that they need to manage the wholesaler (the way I'm now managing them - behave like schoolchildren, get treated that way) and that everything will change if I'm still waiting in January.

I've followed the progress of the shipment into Rotterdam and as I write this it should be on the way back out for the 36 hr hop to the UK. Come January, I'll either be happy or I'll be filing papers to take them to court. All good fun 

Experience of the system in Nov and December :
The low sun seriously impacts the output of the array as expected, and shading on the lowest row of our array doesn't clear until almost 11am. Because each row of the array is a separate string, we see the output increase each time a row comes into full sun as the shading moves down the roof. There is nothing much we could do about this. I knew it would happen and the panels have been wired in a way that best copes with it.

In the last few weeks, we have had endless days of cloud where generation was very small. Our peak output today in low bright sunlight was a little over 3kW vs the peak capacity of 9kW. We had 2-3 good hours enough to put 4kWh into the hot water tank, feed the house and tumble dry a load of laundry. It would have been v useful to have been able to store todays power for essential use instead of "using it up" as heat.

I'll add the monthly generation graphs to the previous post until you have a picture of a full year in the UK.

Solar panels make an awesome ski slope for launching snow off your roof  






Last observation: sadly, my experience of software updateable products has been proven right. I never like to be an early adopter and my experience of microsoft and other companies over a working lifetime are that all suppliers routinely break things because their software updates are inadequately tested. Fronius have proved to be no exception and they introduced two significant errors into my system when I allowed one of their routine updates a few months ago.
The lesson is that once you have a properly working config, "just say no" to subsequent updates.


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## Droogs (28 Aug 2022)

BYD batteries are as I understand it all LiFePo4 they ceased production of Li-ion units in the 1st 1/4 of this year. BYD is the biggest name in the battery industry that nobodies heard of in the real world. They recently become the sole supplier for Toyota's future EV battery needs, from next year they will supply over half the batteries used in Chinese manufactured Tesla model 3s, are the preferred supplier for the Aussie national grid battery storage infrastructure make the only approved city EV taxi and bus fleets for use by Chinese municipalities, 3 cities currently have EV only fleets for all public transport. BYD are launching their cars in Europe this year with the Atto3, personally I cant wait for the Han sportswagen to launch (a 300+ mile 5 series size estate car) which should be here next year with their "Blade" type batteries. They make more EVs than Tesla and out strip Tesla by around 300times in terms of building power storage units for buildings use. They are also the largest supplier to industry of battert power storage in the world. And yet nearly no one has heard of them.


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## Spectric (28 Aug 2022)

A good write up, when you talk of battery strings it reminds me of working with UPS's and the large rooms full of battery racks which were also wired in long strings but in them days we used sealed Hawker Oddesey lead batteries. These systems are in some ways similar to your solar panel setup with the need for conversion to 230 Vac. I would say that using the least amount of conversion has be more efficient, if your panels generate Dc then why convert to Ac to only have to convert back to Dc to charge the batteries? 

In houses of the future you could run extra low voltage Dc circuits to deliver power to all the appliances that are currently powered by Ac to Dc wall chargers and such which again saves in conversion. 

We did have the advantage that these UPS systems were often three phase so by using transformers with both a Delta and Star wound secondary you got a 12 pulse output which allowed for very smooth Dc and less capacitance needed for smoothing and maybe the future will be more electic heating using three phase domestic supplies.


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## Spectric (28 Aug 2022)

Something that contradicts what a lot of people say about Chinese manufacturing is evident in @Sideways post, we all complain about the cheap rebranded Chinese machinery on offer and the problems it can give but as I have said it is just you get what you pay for. The Chinese certainly seem to be in the running with Solar panels and are probably the ones pushing technology forward.



Sideways said:


> I was also interested in the solar panel mounting hardware because one of the real risks I anticipated was poor installation leaving me with a leaking roof and unsecure panels.


When you think of the modern truss roof then you have to question it's ability to also carry the weight of an array of solar panels, a lot of people don't even think about this aspect and end up with issues.

Another area is that of potential lightning strikes, how many people fit surge arrestors in there instalations.


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## D_W (28 Aug 2022)

What's the incentive in the UK for batteries instead of direct grid tie in. Do you have variable ("smart") rates that differ based on the time of day?

In the US where I am, we have the option to use tiered rates, but the incentive is poor- little potential savings and a lot of extra planning. Further from where I live, especially in more progressive areas, rate fluctuations during the day and tiering for total use make solar a lot better on a net cost basis. 

For now, we have net metering where I am - you get a huge benefit to reduce your bill to zero but any surplus generation yields little, so the burden of distribution is on the grid instead of a battery. Solar factor in my region is poor (<65% of the very sunny regions in the US, and 20% worse than areas only a couple of hours east), but panels have become cheap enough that even when they're installed by cost-overladen leasing groups, they're still at least break even with net metering, but wouldn't be with batteries/onsite storage. 

Other interesting side point here is that installed levelized cost for panels is about double rooftop of the cost of installation in commercial arrays. But nobody here likes the looks of commercial arrays, so there's less opposition to rooftops. Across from where my parents live and scattered around in different clusters are 2 1000 acre solar "farms" in an area that 15 years ago, nobody ever thought they'd see. 

I sense in time we'll see incentives for more EVs and incentive to leave the EVs tied into the grid as part of the storage and distribution.


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## Spectric (28 Aug 2022)

I suppose if you just sell the surplus back into the grid then you do not have the expense of batteries and there maintenance but if the grid goes down due to the current crisis with enough batteries you can keep the essentials going. I would suspect a generator would be cheaper than a battery bank large enough to support a house for any duration though.


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## D_W (28 Aug 2022)

Spectric said:


> I suppose if you just sell the surplus back into the grid then you do not have the expense of batteries and there maintenance but if the grid goes down due to the current crisis with enough batteries you can keep the essentials going. I would suspect a generator would be cheaper than a battery bank large enough to support a house for any duration though.



Generators are common here in rural areas. I live in the burbs and have one, but it's only big enough to run either the furnace (gas - just need enough power to run the fan) or the freezer and fridge, and not all three at the same time.

I believe our rate at this point in dollars is about 0.16 per kwhr. I don't have panels (some inlaws do), so I'm not sure about surplus other than to hear that their sentiment was that even the generation rate isn't returned. Of the 0.16 here, I think the generation rate is a little less than half, so if you're generating surplus by accident and the figure is something like 4 cents, it doesn't make sense compared to the installed cost.

It can't be too long before in the US, there are reductions to the net metering as solar has taken off. Both commercial solar and wind are viable without subsidy at this point.


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## Ollie78 (28 Aug 2022)

D_W said:


> What's the incentive in the UK for batteries instead of direct grid tie in.



For me it seems the point of going solar with a battery is to not have to buy electricity from the grid at all if possible.
It seems silly to generate electricity, sell it to the grid at a poor rate, only to have to buy it back again at a much higher rate.
The government's incentives for buy back rates have stopped now, my brother still gets a good rate as he signed up before it changed.
Of course in winter you will have to buy some with short days etc. 

Ollie


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## D_W (28 Aug 2022)

Ollie78 said:


> For me it seems the point of going solar with a battery is to not have to buy electricity from the grid at all if possible.
> It seems silly to generate electricity, sell it to the grid at a poor rate, only to have to buy it back again at a much higher rate.
> The government's incentives for buy back rates have stopped now, my brother still gets a good rate as he signed up before it changed.
> Of course in winter you will have to buy some with short days etc.
> ...



Things must work differently there. They generally won't allow you to install and connect an array that's larger than your proven use here. Things may have changed due to EVs and maybe they allow an adjustment, but I'm not sure. Their objective is to make sure that you're not supplying power back to the grid as a generator. 

So, what happens with the net metering is if you use a certain amount of Kwhr for a month, it doesn't matter when you use it. If you can do the whole thing perfectly, let's say, your bill is zero or maybe ever so slightly negative. If you're using 8kwhr for an oven and giant air conditioner, but it averages out to 2kwhr during the day and your panels generate the same amount, the meter just reports the net. So you're freeriding generated power off of the grid (the same as you would draw from a battery) when you need to and pushing power onto the grid when you don't need it. 

You more or less get to use power like you have a battery but without the battery. The fractional generation reimbursement is, I suppose, to prevent someone from drastically changing consumption and getting paid the same as a preferable generator. As in, a gas generation station is someone you'd rather pay 7 cents an hour to vs. someone's rooftop, but the net metering rules cause the utility to pay the same for power from both, and even further than that, require the grid that has supplied some power and taken some from the panels to suddenly remove the grid charge, too. 

That part doesn't make as much sense and it's changing (and should). 

Before batteries, I remember standalone systems having coil shunts and such things. It'd be dandy if those shunts heated water and then heated a fluid mass somewhere for fall and winter heat, but I'm sure that's not a feasible thing.


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## Sideways (28 Aug 2022)

The sand battery that has gone live in Finland recently stores renewable energy as heat ( 600C) for months, to feed back into an underground district heating system in the winter. Simple and inspiring.
It would be awesome to see a rural domestic version of that. A 10x10x10 foot cube of sand would take all my excess summer energy to give back as heat in winter. Intense sources energy are more useful than dilute ones. That's why solar PV is more versatile than solar thermal, and the sand battery has advantages over hot water buffer tanks provided it's own challenges can be solved.


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## Sideways (28 Aug 2022)

pe2dave said:


> @Sideways , you say "First is to use an AC coupled battery which simply wires in to any new or existing system using 240V AC."
> I don't understand that idea, explain please?
> 
> Excellent and thorough view, much appreciated.


Dave, I'll come back to this when I figure out a better way to explain it clearly. I won't forget but bear with me a day or three.
I'll try and complete my "place holder" posts as quickly as possible because I have learnt that once a post is made, you only have 24hrs to go back and edit it.
After that, Mike K will be helping me to bully this into a useful and hopefully readable thread 
Cheers Mike !


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## Lorenzl (28 Aug 2022)

I suppose the other thing is the grid doesn't not need your surplus during the day. They would prefer it at night hence the buy in tariff is low?


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## Inspector (28 Aug 2022)

Mike perhaps one of the Phase Change Materials could be used to store heat for later use.






Phase change materials for thermal energy storage | Climate Technology Centre & Network | Tue, 11/08/2016


One of the disadvantages of modern lightweight construction is its lack of thermal mass, which means this type of building can overheat in the summer and can’t retain heat in the winter. Often, heating and cooling systems are installed to maintain temperatures within the comfort zone. However...




www.ctc-n.org










Phase Change Material (PCM) Selection | PCM Technology


There are several factors that need to be considered when selecting a phase change material. ACT uses the optimal PCM in designing and manufacturing your custom heat sink. Contact a thermal management expert now to discuss your application.



www.1-act.com





Pete


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## Lorenzl (28 Aug 2022)

@Sideways there is one on the way Caldera but only for re heating water.

I am thinking about installing a Sunamp phase change battery.


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## Sideways (28 Aug 2022)

I like the Sunamp phase change concept. Especially their research into why these materials typically fail after a small number of cycles and their proprietary modifications to a well established material that is claimed to extend its life to over 30 years.
Right now though, they are using this to solve the wrong problem. The largest domestic battery is equivalent to a 300 litre water tank, at greater cost, but in less space. It is about freeing space in small homes.

If they made a unit as big as a 300L water tank that held equivalent of 600 L of energy, I'd be interested.
Also, the Sunamp gives it's energy back at 55C. A hot water tank can store it's whole contents at 70 odd and be diluted when used. That increases the effective volume of hot water that you can store in a simple tank.

Sadly, none of these devices are able to store enough water, hot enough, to maintain central heating overnight.


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## Robbo3 (29 Aug 2022)

Very good write up.


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## Denty (29 Aug 2022)

Sideways said:


> *MyEnergi smart controls*
> 
> There is little argument that the way to get maximum value from solar PV is to maximise self consumption.
> Key to this are
> ...


Very informative and might I say timEly, thank you


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## MikeJhn (29 Aug 2022)

A123 in the USA where the originators of the LiFe (lithium nano phosphate) battery about thirty-five years ago, then shortly afterwards came the Chinese copy PO4 and as far as I now they are both Lithium Iron batteries, is it me or others that are confused, if someone could explain the chemical difference that would help my old brain get less cloudy.


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## Jacob (29 Aug 2022)

Phew a lot of information above!
Battery seems to be the weakest link in so many of the alternative energy schemes, except as back up, which is how they've always been e.g. to _start_ an ICE and keep it going but not to _run_ one (very far).
Maybe low tech energy storage is the way? Hot water tanks? Pumped water for hydroelectric generation?
Or to to link all solar panel installations to a bigger grid (national?) and completely separate generation from distribution?
Ditto EVs - having lower tech replaceable batteries available at charging stations rather than aiming for long journey self sufficiency - or simpler still, have direct electric pick-up as per rail, tram etc?


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## Jacob (29 Aug 2022)

Sideways said:


> .....
> 
> Sadly, none of these devices are able to store enough water, hot enough, to maintain central heating overnight.


But luckily hardly anyone needs central heating overnight. Those that really do need it become another design problem, possibly involving underwear (remember the dressing gown?), or small and well insulated rooms?

Thinking out of the box required - two particular boxes may need a lot of rethinking; personal habitation and personal transport.

Just reading Adventures in the Anthropocene by Gaia Vince – review
Essential reading, with lots of accounts from around the globe of people who are already dealing with the effects of climate change, but largely in temperate/hot (too hot) regions. EVs don't get a mention!


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## NormanB (29 Aug 2022)

Sideways said:


> We live in Cheshire, UK
> I have been very happy with my installer. For now let's protect their reputation in public from any of my misdeeds  PM me if you are in this part of the world and still want to know.
> I think "in roof" is great. It looks smart and would be the way to go with a new build or a reroof. Two caveats though. First is that mounting solar panels flush with the roof means little or no ventilation behind them. Solar panels get hot and generation is reduced when they are hot. So in roof is electrically less efficient because the panels run hotter. I haven't quantified this. Second is that some in roof systems use or require specific panels. The in roof systems on show at the trade show I mentioned all used panels that were just a mid range PERC technology. My preference for the high spec N type panels wasn't negotiable.
> Victron equipment looks excellent to me. They have a very good reputation for off grid solutions and the equipment appears well made. I'd recommend anyone interested to consider them.


Thank you. Sadly am not in your area.
Yes I was aware of the performance hit with in roof, just for me - aesthetics outweigh that.
My research continues (but I am favouring Victron as they seem to have the system integration sewn up).


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## Sideways (29 Aug 2022)

A couple of new housing estates that I have seen develop over the last 5 years both include a dozen or more houses built with in-roof panels. They look very tidy. 

Our home is 40 years old and roofed with Marley cement tiles. I queried Marley about the expected life of their tiles and heard that 60 years even 70 year life is not uncommon. It felt just a step too far to add in a roof replacement to our project.

If I had the ££, I would have been tempted.


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## Dabop (29 Aug 2022)

The BYD BBox is quite popular here in Australia, a modular storage system that can be easily expanded later if you find you need more storage, and considerably cheaper than the Powerwall from Tesla (plus as the BBox uses LiFePO4 cells, it basically lasts about twice as long as the Powerwall with its lithium-ion cells





BYD is a HUGE player in China, their production levels are equal to Teslas 'gigafactories', and they have been available for about the same time (but only recently have they started looking at external markets- all their production was sold inside China- mostly to their automotive arm...)

BYD made the 'ElectricBlu' city buses, first trial in Australia was in 2016 for a 400km electric bus with first large order in 2017, now they are looking at replacing almost all the diesel buses with them in Sydney




They have been selling a fully electric BEV, the E6, since 2009, widely used in China as taxis (I got a ride in one here in Sydney, and they are a very zippy little beast)




They are well in advance of Tesla in one aspect, you can't buy a Tesla truck or semi trailer- walk in, plonk down the cash and drive out in a BYD electric truck....




They don't make 'supercars' like the Teslas- they are more the 'Toyota Corolla' of the BEV market- but they do have a long history in it- as long as Tesla has been doing it...


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## sumo2001 (30 Aug 2022)

Very interesting read


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## Jaydee (30 Aug 2022)

Yes a very interesting read. Thank you for spending the time writing it up.


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## NormanB (30 Aug 2022)

Sideways said:


> A couple of new housing estates that I have seen develop over the last 5 years both include a dozen or more houses built with in-roof panels. They look very tidy.
> 
> Our home is 40 years old and roofed with Marley cement tiles. I queried Marley about the expected life of their tiles and heard that 60 years even 70 year life is not uncommon. It felt just a step too far to add in a roof replacement to our project.
> 
> If I had the ££, I would have been tempted.


As I understand it the in roof is fitted in/on a plastic tray which has suitable profiles to accommodate the profile of common roofing tiles. So no need to replace the roof, just the requisite number of tiles - rather like fitting a Velux window.


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## NormanB (30 Aug 2022)

Sideways said:


> *MyEnergi smart controls*
> 
> There is little argument that the way to get maximum value from solar PV is to maximise self consumption.
> Key to this are
> ...


Thank you for this and it is always interesting to see detail.

Just one think have you considered the arbitrage potential using a battery and the Octopus Agile tariff?

Here is a screenshot of my sons installation:




Note the price difference between export and import. The arbitrage can be between 20p per KWH and 45p per KWH

Clearly he is charging his battery from solar or by importing it and exporting to maximise arbitrage. He is still refining his controls and timing for best effect but is already yielding impressive results.

Not sure how long this ‘situation’ will continue.


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## Dibs-h (30 Aug 2022)

NormanB said:


> Thank you for this and it is always interesting to see detail.
> 
> Just one think have you considered the arbitrage potential using a battery and the Octopus Agile tariff?
> 
> ...


I know of a chap that fitted a 100kWh battery, charges it between 00:00 & 05:00 on Octopus's economy 7 tariff, at 5am the house disconnects from the mains and powers his house till 00:00. Then rinse and repeat. His usage is around 11kWh\day but from what I've read, by not deep discharging the battery much, the lifespan is longer.

Found it very interesting especially as phase 2 for him is the addition of PV panels.


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## Deadeye (30 Aug 2022)

We've had a 4kWp system on our east facing roof for 8 years now. We get feed in and generation payments at c. 5p and 17.5p respectively. The system cost £6k. It's on an east facing roof in Buckinghamshire and we're on track to break even this year in cash terms. We have a basic diverter box that spots export and heats the thermal store instead but I don't have a way to value that. There's a second tier outlet to a radiator in the cellar in winter.
Couple of chip-in comments:
Firstly, panels do better when cool so east better than west if that's your choice.
Secondly, the diverter box (£400 I think) is brilliant and has got us used to running the washing machine etc. More when it's sunny.
Thirdly, I was worried the efficiency would decline over time, but the payments year to year are steady.
We don't have an electric car yet (but we don't fly or eat red meat) so can't use PV to charge, but overall couldn't recommend it highly enough.
In a roundabout connected way, I'd also recommend loft stilts and an extra 170mm of rockwool, and just checking hot water pipe routing. Our shower is under the thermal store but used to go the length of the house and back!


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## D_W (30 Aug 2022)

Dibs-h said:


> I know of a chap that fitted a 100kWh battery, charges it between 00:00 & 05:00 on Octopus's economy 7 tariff, at 5am the house disconnects from the mains and powers his house till 00:00. Then rinse and repeat. His usage is around 11kWh\day but from what I've read, by not deep discharging the battery much, the lifespan is longer.
> 
> Found it very interesting especially as phase 2 for him is the addition of PV panels.



Different battery type, but eons ago now, the toyota prius drew a lot of fire for "how much will it cost to replace the battery when it no longer works". 

The figure was something like $3k - the article writer interviewed a battery supplier who conveyed that most of their sales were for repairs, not end of life replacement. The batteries never cycle deep in the older prius and they never really wore out. 

Repairing the charging system on them or any of the others in the US was a different story, though. Shorter life than the battery and more cost to fix.


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## pe2dave (30 Aug 2022)

How long are Octopus rates good for?


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## Dibs-h (30 Aug 2022)

pe2dave said:


> How long are Octopus rates good for?


No idea Dave. What interested me was that he used the battery pack out of a BMW i3, whilst others had used the packs out of Teslas and Leafs (I think).


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## 1steven (31 Aug 2022)

Very i


NormanB said:


> Thank you for this and it is always interesting to see detail.
> 
> Just one think have you considered the arbitrage potential using a battery and the Octopus Agile tariff?
> 
> ...


Very interesting graphs have been looking for realtime information for a while


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## point5clue (31 Aug 2022)

Presumably the shift to variable renewable energy generation (both commercial and domestic) will incentivise energy companies to offer variable rates that encourage consumption when there’s an excess of available power and discourage it when it is scarce.

I’m wondering if the long-term economics of having a domestic solar PV system will be reduced by this trend, and I guess on the flipside the economics of having a battery or even just simple energy diverter to a tank or pile of bricks will improve.

This may be just me rationalising the fact that I don’t have spare cash to buy any of these systems at the moment.

I think in the future a sweet spot for many people will be an electric vehicle with V2G that can both arbitrage the energy system for the house and charge cheaply itself. It remains to be seen if any of this becomes a reality before the sea level rises take us back to the Stone Age


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## Sideways (31 Aug 2022)

NormanB said:


> Thank you for this and it is always interesting to see detail.
> 
> Just one think have you considered the arbitrage potential using a battery and the Octopus Agile tariff?
> 
> ...


This is a very appealing idea. When I first started seriously considering solar + battery and looking at tarrifs, I had a spell of considering a large battery and no solar. The issue is battery life. It very much feels like a roll of the dice just how long a battery will last (#cycles x depth of discharge, not years). 
My battery (will be) a huge part of the financial investment.
I've researched as best I can about it but really don't know. This is much the biggest unknown in the whole project. It may mean I've made a superb long term investment or perhaps just "prepaid" my next 20 years electricity bill in one go.

If you calculate the economics based only on the warranted life of the battery to eliminate that uncertainty, this DID NOT look like a way to profit when I was looking at rates, but it is early days and tarrifs change.

Consider:
£10,000 battery
Will it achieve 10,000 cycles ? Maybe. At 1-2% capacity loss per 1,000 cycles, that would be tolerable. It's not unreasonable to hope for this.
So £1 per cycle, and some arbitrary average of 12kWh per cycle so as not to fully cycle it and extend the life
That's 8.3p per unit of your arbitrage needs to be set aside to pay for wear and tear on the battery (replacement after a nominal 10,000 cycles)

We just need to be careful, go into this with eyes open and pay a lot of attention to battery reliability and lifetime data. Household or V2G.

I've seen just one public domain data set on my specific model of household battery, it's encouraging, but no one is sharing data about tests out at 10,000 cycles so it's all just extrapolation from the first 1000, 2000, 3000 cycles.


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## Inspector (31 Aug 2022)

Until 4 years ago there was a magazine published in the US called Home Power. There downloadable copies of all the issues for free as long as it is for personal use. While it may not have the latest and greatest information there was a lot of good information. Home Power Magazine | Downloadable Archive | Home

I stumbled across a wind and solar forum in the US, there must be others. Another place for information that would be current. Northern Arizona Wind & Sun Solar Forum

Pete


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## Sideways (31 Aug 2022)

Have a look here:



Reports – Lithium Ion Battery Test Centre



This was a 3 year project in Australia
Report #12 was the last, it's now finished.

My own on-order battery the BYD HVM is among those. It was cycled 1100 times - once a day for almost 3 years. Degredation wasn't measurable as a firmware upgrade actually increased it's capacity along the way, but it's curves like the ones in this report that are needed to really judge how good a battery may be.

Notice how rather a lot of batteries had early failures or issues that needed the manufacturer to be called in.

Notice also - and not picking on them, merely using a well known name to make the example - the performance data collected in this trial suggested that their Tesla Powerwall 2 battery pack would be down to 60% capacity in less than 5,000 cycles. 

I don't think it's safe to assume that batteries will have an infinite life. You need to consider that they may degrade and need replacement in as little as 10 years In my case that could be £1,000 / year, making the whole project more break even than profitable.


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## Phil Pascoe (31 Aug 2022)

point5clue said:


> It remains to be seen if any of this becomes a reality before the sea level rises take us back to the Stone Age


It's likely to be a long time - the ODN (Ordnance Datum Newlyn) hasn't changed in the last hundred years.


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## Jameshow (31 Aug 2022)

Sideways said:


> This is a very appealing idea. When I first started seriously considering solar + battery and looking at tarrifs, I had a spell of considering a large battery and no solar. The issue is battery life. It very much feels like a roll of the dice just how long a battery will last (#cycles x depth of discharge, not years).
> My battery (will be) a huge part of the financial investment.
> I've researched as best I can about it but really don't know. This is much the biggest unknown in the whole project. It may mean I've made a superb long term investment or perhaps just "prepaid" my next 20 years electricity bill in one go.
> 
> ...


What size battery do you recommend 5x 10x daily usage?


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## emlclcy (1 Sep 2022)

Sideways said:


> Have a look here:
> 
> 
> 
> ...


hi, very interesting, can you elaborate on your system a little more pls?
regards
carl


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## emlclcy (1 Sep 2022)

hi, very interesting, can you elaborate on your system a little more pls?
regards
carl


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## NormanB (1 Sep 2022)

pe2dave said:


> How long are Octopus rates good for?


They are half hour pricing intervals


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## NormanB (1 Sep 2022)

Sideways said:


> This is a very appealing idea. When I first started seriously considering solar + battery and looking at tarrifs, I had a spell of considering a large battery and no solar. The issue is battery life. It very much feels like a roll of the dice just how long a battery will last (#cycles x depth of discharge, not years).
> My battery (will be) a huge part of the financial investment.
> I've researched as best I can about it but really don't know. This is much the biggest unknown in the whole project. It may mean I've made a superb long term investment or perhaps just "prepaid" my next 20 years electricity bill in one go.
> 
> ...


Yes I agree on the life of the batteries. For modern battery technology, using the battery to support the normal domestic load is a gentle affair and charging from solar is similarly gentle. However importing to charge up the battery from a low state of charge to maximum and then discharging it ‘rapidly’ to the grid is probably more detrimental to battery life.

I also cannot get my head round the ‘huge’ arbitrage available at the moment so cannot figure out if this in an aberration or sustainable in the long term. I guess for ordinary mortals they will not be investing in significant battery capacity and most will not invest at all, so in grid terms the % of homes with an excess of 10KW/hr to pump back into the grid is probably less than 1%. However, post 2030 when EVs gain a significant share of personal transportation AND it is all V2G capable (including the DNO infrastructure) with the potential for individual 50KW/hr inputs to Grid at peak times - this could be a paradigm shift for the Grid and will influence price (lower the available arbitrage - I think)

To be honest - ‘it does my head in’ - getting too old for this.


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## Cooper (1 Sep 2022)

Phil Pascoe said:


> It's likely to be a long time - the ODN (Ordnance Datum Newlyn) hasn't changed in the last hundred years.


Not according to the OS. From their website:
While ODN was a measurement of mean sea level in 1915-1921, it's important to know that mean sea level has changed since then – it even varies throughout Britain depending on where you are measuring! So, it's best to think about ODN as a height reference point, a measurement of sea level at a specific point in time at a precise location.

I am fascinated by the reasoning involved in this discussion. I'm not sure what the average value of the homes of contributors to this column is but lets say that a current valuation is £500,000. In very recent times there has been rise in value of 10 or 11% so a home worth £450000 is now worth £500000, just by being a house. A sum considerably more than the cost quite a lot of solar panels etc, external cladding and possibly a contribution to a heat pump. In the same period a very nice motor car bought about a year ago that cost the same as the increase in the value of the house, is now worth several thousand pounds less. 
It seems that an investment that is contributing to reducing carbon emissions, reducing our dependence on fluctuating energy prices and making the home considerably more comfortable has to show a monetary profit, irrespective that the value of the home increasing. While the flash car can merrily sit on the drive and lose a few thousand a year (in some cases a sum equivalent to the energy costs of a home) but it is considered normal economic activity. 

I realise that most of us here aren't in a position to splash out 50K on a flash motor but looking at the number of newish SUVs I see around, lots of people are. Those cars are harming the future of our children and grandchildren, investing in clean energy and reducing dependence on fossil fuels makes a contribution to reducing the harm.

Funny old world.


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## Phil Pascoe (1 Sep 2022)

Cooper said:


> While ODN was a measurement of mean sea level in 1915-1921, it's important to know that mean sea level has changed since then – it even varies throughout Britain depending on where you are measuring!



The height is still measured in Newlyn, and has been for over 100 years ......... in which time it hasn't changed (according to another of their sites).


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## bansobaby (2 Sep 2022)

Phil Pascoe said:


> It's likely to be a long time - the ODN (Ordnance Datum Newlyn) hasn't changed in the last hundred years.


The ODN datum level hasn't changed because it is the average of a large number of sea level readings taken over a period of several years up to about one hundred years ago.

It is not related to current sea levels, which have changed since the ODN was established....


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## bansobaby (2 Sep 2022)

Phil Pascoe said:


> The height is still measured in Newlyn, and has been for over 100 years ......... in which time it hasn't changed (according to another of their sites).


It's a fixed datum, which is now underwater at high tide.....


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## moosepig (5 Sep 2022)

Sideways said:


> LiFePo4
> They haven't landed yet but it will be a BYD HVM battery stack.
> 7 modules of 2.x kWh apiece making 16kWh
> It looks like a stack of shoeboxes and stands about 1.6M tall x a couple of feet wide.


Thank you for this thread, following with great interest. Especially keen to learn how your battery performs when it finally gets added.

Are there limitations in where the inverters, battery, isolators etc can be installed? For example could they be be put in a (fully accessible and boarded) loft space? I'm looking for a solar solution, similar to yours with a dc-coupled battery, but I have no garage and realistically loft space is the only part of the house that has room. The panels would be on the roof anyway...

Also will any SMETS2 smart meter work with these setups, or is this something that the installer should be recommending? I'm very keen to end up with a system that will continue to power the house during a power cut BUT be intelligent enough not to try to export while the grid is off (I'd rather not fry the grid repair team!); is the choice of meter part of this aspect of the design? 

Thanks again, brilliant thread!


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## Sideways (5 Sep 2022)

It is very common to have the inverter installed in the loft, but my size of batteries, probably not. They weigh a lot. A 5kWh wall mount battery is much smaller and lighter and wouldn't be an issue.

You just have to be aware of temperature with electronics and batteries. Too hot and they could be stressed, cooling fans running all the time. Too cold and batteries suffer. You might need to box around the batteries and include a fan and a greenhouse heater to control temperature extremes.

My kit is in the garage for these reasons close to the consumer unit.

The fronius GEN24 inverters coupled with a battery will run when the grid fails. It's a feature of these high tech inverters. Either powering a single socket for manual emergency use or (with a complex and probably expensive bank of specialist relays) they can do auto switchover of as many circuits as you want subject to the discharge capacity of the battery.
There is incidentally, an absolute obligation not to back feed the grid from your solar system. You and your installer don't have a choice in this and to prevent it the majority of inverters simply won't work when disconnected from the grid, battery or not.

I will experiment with this - keeping lights, boiler and the IT on during a power cut is nice to have but honestly we don't go off very often and it wouldn't be worth the hundreds that it would cost to build the switching circuits for auto changeover and prevent any chance of feeding back into the grid.
A simple socket will probably do me.

As for meters. Fronius GEN24 inverters will do export limitation. Mine is setup that way. A Fronius smart meter is needed to control that. Export limitation means that the inverter will automatically limit the amount of power it tries to feed into the grid - necessary sometimes because the grid can't absorb it without (for example) the localised voltage on the grid being pushed too high. In the summer, this feature was working a lot. Our generation was often several kW and whenever we weren't using enough ourselves, the inverter throttled back to limit the amount we put into the grid to no more than a basic 4kW system would.

There's another electronic meter installed - a standard device to keep a simple record of total generated power.

We don't yet have a smart meter. That was a disgraceful affair if you read about the way the energy co's mismanaged the entire project and wasted £100's of millions, so I've refused one to date. Now they are finally getting a grip, I will let them put one in once my batteries are installed and I'm happy with everything else.
A smart meter is being made a condition of access to ev tariffs and others that vary by time and day of the week so they are inevitable in the long run.

_(We made 26kWh today)_


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## Terry - Somerset (5 Sep 2022)

Fundamental to the economics of domestic energy generation and storage is the long term grid price, complicated by different charging policies - eg: day and night rates. 

Investment cost of PVs and battery storage has reduced steadily over the past decade. Costs are likely to be relatively more stable than grid costs influenced by world market fossil fuel prices. 

Cheap rate (eg: Octopus) + battery storage to offset higher daytime prices may be a short term advantage. If households increasingly install this technology, surplus capacity in the network will reduce making cheap off-peak rates less plausible.

PVs without storage rely upon a high level of usage during the day - surplus generation can only be exported to the grid at uneconomic rates.. When the sun goes down they generate no power. 

Separate batteries need to be sized appropriately depending whether the intent is (a) to store unused daytime PV output for use at night, or (b) provide off grid resilience - potentially expensive for little gain given a largely reliable grid. 

The medium term optimal solution will be using PVs to charge EVs and using EV batteries as the means of energy storage. This is likely to become the default as EVs become mandatory.

Until recently an investment which generated a return of (say) 7-10% would have seemed pretty good (a payback 10-14 years). This was very broadly the intent of the now ceased feed in tariffs to encourage uptake of PVs. 

A long the payback period, makes the volatility of the assumptions critical. I would be inclined to install PVs only when I have an EV or plug in hybrid - this will ensure that I make best use of my investment irrespective of grid costs.

However all our circumstances are different - retired, out at work all day, at home with young children, commute long distances, have driveway, live in a flat, etc. There is no one size fits all - and for those as, or more concerned about the environment it is not just a financial choice.


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## Dabop (6 Sep 2022)

I run a (temporary) offgrid system here to power my shed and caravan (still building the house) and have been in the solar game here in Australia since the 1980's (offgrid) and gridtie (2004)
Panel life is expected to be probably half a century or more- my first own offgrid house has its panels are still in use at an ex neighbours and they will be 40 years old next year- and still producing over 70% of their original output so panel life really isn't a concern- they do gradually lose a bit of output each year, rather than (as some people expect) put out the same output- most panels have a minimum of 82%-85% of their original output at 25 years, some are over 90% at 25 years, but cost more

The battery bank is a 48v 400Ah LiFePO4 system (20kwh), using 16 Winston cells- I have been using them in offgrid installs for about a decade now, and they have a very good rep here in Australia, been on sale in Oz since 2008 and so have a good history- they guarantee a minimum of 5000 cycles at 80% DOD (12-15 years), 7000 plus cycles at 70% DOD (20 years), mine run at 30% DOD and will likely blast past 10000 cycles (25 years plus)- and cost me less than a Tesla powerwall which will be well down on its capacity in only 10-12 years (they currently use lithium-ion cells, rather than the LiFePO4 cells I used)
My total system cost was under $18k Au- the battery bank was $11.5k AU, the 12kw continuous, 36kw peak inverter was about $3k, the solar panels (18kw, 72 x 250w secondhand ex gridtie panels) just under $2k, the solar controllers $400 and the mounts about $500
It made economic sense to go offgrid here- getting the mains on was quoted $42k- and that was a 8kw limit on it to boot (SWER out the front and transformer needed)- and being a rural spur- meant frequent blackouts (often several times a week!!!) and bills as well- be mad not to go offgrid lol
So my system paid for itself from the first day...


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## Mal-110 (6 Sep 2022)

The idea of using batteries of electric vehicles to feed back electricity to the grid in times of need is not a possibility at the moment. This would fall under the SEG (smart export guarantee) scheme, and you may need a micro generation certificate. However the main problem is that the energy companies do not accept energy from batteries which they term brown energy. You have to prove when you accept the SEG that you will not send them energy from your batteries. This will have to change as will the SEG payments which are between 3p to 7.5 p per KWH, when the average cost of a KWH or electricity now costs 26.9p plus vat. A profit of 500% on average.


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## NormanB (6 Sep 2022)

Mal-110 said:


> The idea of using batteries of electric vehicles to feed back electricity to the grid in times of need is not a possibility at the moment. This would fall under the SEG (smart export guarantee) scheme, and you may need a micro generation certificate. However the main problem is that the energy companies do not accept energy from batteries which they term brown energy. You have to prove when you accept the SEG that you will not send them energy from your batteries. This will have to change as will the SEG payments which are between 3p to 7.5 p per KWH, when the average cost of a KWH or electricity now costs 26.9p plus vat. A profit of 500% on average.


Is all of that a universal truth in the UK and all DNOs?


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## D_W (6 Sep 2022)

Mal-110 said:


> The idea of using batteries of electric vehicles to feed back electricity to the grid in times of need is not a possibility at the moment. This would fall under the SEG (smart export guarantee) scheme, and you may need a micro generation certificate. However the main problem is that the energy companies do not accept energy from batteries which they term brown energy. You have to prove when you accept the SEG that you will not send them energy from your batteries. This will have to change as will the SEG payments which are between 3p to 7.5 p per KWH, when the average cost of a KWH or electricity now costs 26.9p plus vat. A profit of 500% on average.


 I guess a question is what is the actual grid charge vs. generation. 

Our charges are itemized here - we can choose different generation options if we want to, but they're all about the same because it costs about the same for the producers to make energy the same way as the others. 

In our case, the generation is about 8 cents to the grid, and the grid charges (the cost to maintain and improve the grid) are about the same. 

We have net metering and I haven't heard of anyone feeding battery back into the grid yet, but the net part means that you get a 100% offset for amounts up to what you use, even if you draw hard from the grid and then feed back to the grid - it's free riding. 

Once you're over the limit, the actual payment in most states is a fraction of the generation cost because instead of being a generator contracting with the utility to meet needs, you're just tossing electricity onto the grind however you'd like and it may or may not be wanted or needed. Things will change here at some point because the generation is "unregulated" (it really isn't) but providing transmission and grid access is highly regulated and if there's an actual need to increase revenue to cover the cost of the grid, the utility regulator will just approve it.


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## Sideways (6 Sep 2022)

For the data fiends, I've put up a bunch of generation graphs in the post that I reserved near the bottom of page 1. It gives a feel for the way output varies day to day with the weather and the performance through the summer months.

These graphs come from the Fronius web.
August is slightly low as the inverters were switched off and covered up for the last couple of good days while I replaced the roof of my garage. I didn't want debris falling into them !

"When can we have Eddi back ?" started after the first day. We quickly get used to having new tech !


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## Terry - Somerset (6 Sep 2022)

There may be a disconnect between that which is good for the consumer vs the energy companies.

Assuming:

Battery storage (fully utilised) ~15p kwh.
PV ~10p kwh. 
Price cap 52p.
SEG ~6p kwh.
Using a battery installation as a means to store energy generated by PV maximises the benefit of a PV system. Total cost is ~25p kwh, about half the current price cap.

Consumer investment in more capacity than needed domestically is pointless. SEG at ~6p kwh is less than cost, but does generate some income for unavoidable excess production.

Justifying investment in batteries simply to charge at off peak rates and sell back to the grid only works if SEG is more than the cost of batteries. Current wholesale electricity price of ~25p kwh means this is unlikely or marginal.

Energy suppliers have no interest in increasing SEG at the expense of their profits. It may even be a circular argument - increasing SEG may increase the price cap to make it affordable. There may be technical issues of which I am unaware which makes SEG complex anyway!

For UK PLC - consumer investment in PV and batteries, and allowing energy to be fed back to the grid, would limit peak demand and reduce the need for investment in additional capacity.

Overshadowing all this logic is that the technology is very fluid and rapidly changing. In a free market peak and off peak rates may equalise as the Internet of Things could allow:

flexible charging in nano-second intervals drawing energy stored in EVs and batteries
integrating with output from solar, wind, and power stations,
all whilst non-critical appliances and users shed load for short periods
As consumers we should be able to select criteria to be used so that we make individual choices to meet personal requirements.


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## Dabop (6 Sep 2022)

One difference in the way batteries are used in Australia with gridtie systems is that they are used more for self powering your house at night, and only if the demand is too high for the battery bank or they are depleted does the house switch back to the grid, and in the mornings the PV array prioritises recharging the house bank first, EV if present second after which any excess is then available for export

So sort of like a 'semi offgrid' but using the grid as a 'backup genny' and with the ability to export, rather than 'waste' any excess PV capacity- this means for many, they are basically self sufficient for 99% of the time (friend of mine has such a system, and only during winter did he actually use the grid for some of the time for heating (reverse cycle aircon)- his total bill for the year was under $200- most of which was fees for the connection...)


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## Mal-110 (8 Sep 2022)

NormanB said:


> Is all of that a universal truth in the UK and all DNOs?


If you intend to generate more than 3.68kwh then you need to ask permission from from your DNO (Distribution Network Operator) showing how you intend to wire up the system. No permission required under this but you have to tell them. Most new systems will need permission which could be granted for free but not necessarily as each DNO has differing rules. Also the DNO could advise a different wiring regime. You could then generate electricity and all would be good. If you want to sell your electricity you now need an SEG (Smart Export Guarantee) which is an agreement you make with a chosen electricity supply company OVO, Octopus etc. This agreement includes the proviso that you will not send brown energy or energy from a battery to the grid and you must provide evidence your system won't do this. Most installers know this and can provide the evidence to demonstrate compliance. 
I am not an electrician but it seems to me that the DNO's are rightly very concerned that electricity remains in batteries in the event of a power cut and not feed back into the grid. 
The electricity companies offer differing and in my view miserly rates per KWH. You don't have to use the same company that you buy electricity from to sell to. In our case we are with OVO and have applied for the SEG from Scottish Power. 
Is my assertion Universal in the UK? well in my research to find a better rate it would appear that all seven I checked had a clause prohibiting brown energy. There may be a company that does not but I have not found it, but then again I was looking for more than 4p per KWH. 
My thoughts are that this clause was included to prevent the possible but extreme scenario of filling your battery up at a low tariff rate and selling it back to another company during high peak times. This won't happen at the moment because of those miserly rates but could in the future.
As far as using EV batteries to feed the grid then in they are no different than home batteries in relation to the effect on the grid
The DNO looks after the network and the electricity companies look after our bills.

Hope this explains it.


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## Dibs-h (9 Sep 2022)

Mal-110 said:


> If you intend to generate more than 3.68kwh then you need to ask permission from from your DNO (Distribution Network Operator) showing how you intend to wire up the system. No permission required under this but you have to tell them. Most new systems will need permission which could be granted for free but not necessarily as each DNO has differing rules. Also the DNO could advise a different wiring regime. You could then generate electricity and all would be good. If you want to sell your electricity you now need an SEG (Smart Export Guarantee) which is an agreement you make with a chosen electricity supply company OVO, Octopus etc. This agreement includes the proviso that you will not send brown energy or energy from a battery to the grid and you must provide evidence your system won't do this. Most installers know this and can provide the evidence to demonstrate compliance.
> I am not an electrician but it seems to me that the DNO's are rightly very concerned that electricity remains in batteries in the event of a power cut and not feed back into the grid.
> The electricity companies offer differing and in my view miserly rates per KWH. You don't have to use the same company that you buy electricity from to sell to. In our case we are with OVO and have applied for the SEG from Scottish Power.
> Is my assertion Universal in the UK? well in my research to find a better rate it would appear that all seven I checked had a clause prohibiting brown energy. There may be a company that does not but I have not found it, but then again I was looking for more than 4p per KWH.
> ...


Do you still need permission if your array is (or will be) greater than 3.68kwh but won't be feeding into the grid?

Cheers


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## Mal-110 (11 Sep 2022)

Dibs-h said:


> Do you still need permission if your array is (or will be) greater than 3.68kwh but won't be feeding into the grid?
> 
> Cheers


Not such an easy answer as it happens. In domestic applications, if you have a solar array and your house is on the grid you must inform the local DNO under 3.68KWH or get permission if over, irrespective of feeding back. If you are completely off grid, well the advice is unclear but seems to me that it is not the responsibility of the DNO. The crux is whether or not property is on the grid. In our case I needn't have registered for the SEG but our excess electricity would still be fed back to the grid and we wouldn't get a penny for it. Most installations in the UK will be grid tied and as I understand it this evens out the vagaries of the solar input and assists the battery charging regime.


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## Dibs-h (11 Sep 2022)

Cheers Mal.

The property is currently on the grid and will remain on it. 

The solar would be for "self consumption" with additional loads fed from the grid when the solar and\or battery isn't enough. In summer - would rather "dump" the excess generation into an immersion heater or 2. After that no idea tbh.

Have come across mentions of G98 & G99 (or could be G99 & G100) forms and the kw rating doesn't appear to be for the array size but dependent on the inverter rating.

Cheers

Dibs


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## Mal-110 (2 Oct 2022)

Dibs-h said:


> Cheers Mal.
> 
> The property is currently on the grid and will remain on it.
> 
> ...


Hi Dibs,
Missed your post initially. The forms are for “Notification” and application. My installer filled mine in so not sure which number they are. Some DNO’s charge if they have work to do for the application. You are right the rating is what comes out of the inverter and not the array. The inverter is rated by output, irrespective of what the array produces. However it is better to match them, we have a 4.8kwh array and a 5kwh inverter. So in theory we can only put a maximum of 5kwh back to the grid. The inverter acts as a gateway managing the output. I have seen our array produce 5.11kwh for a short period and I really don’t know where the excess goes. You can only dump so much excess into your water cylinder, so the SEG is the choice of the excess in summer but the rates are miserly.


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