One family's solar story

[Sideways]

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)

 

[Terry - Somerset]

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.

 

[Dabop]

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...

 

[Mal-110]

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.

 

[NormanB]

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?

 

[D_W]

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.

 

[Sideways]

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 !

 

[Terry - Somerset]

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.

 

[Dabop]

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...)

 

[Mal-110]

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.

 

[Dibs-h]

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.

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

 

[Mal-110]

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.

 

[Dibs-h]

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

 

[Mal-110]

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

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.

 

[Sideways]

For anyone still following this thread, I've added updates into to post #13 at the end of 2022 and just now in Jan 2023 to detail the installation of the BYD storage battery which arrived a few days ago. Installation went without a hitch and at first glance, interoperation between the Fronius and BYD components is as good as I hoped for.
It is interesting to note that Fronius - mfr of my inverters - I'm told are now quoting delivery dates of 2024 for new orders placed now ! Despite increasing their manufacturing capacity they can't satisfy demand.

Thanks for reading, for your comments and best wishes to all for 2023.

 

[JimJay]

I'm certainly following it

 

[Sideways]

@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.

I told @pe2dave that I would answer his question about AC coupled vs hybrid DC battery storage in "a few days". Well it's been nearly 9 months so I had better have a go !

Solar panel arrays output a DC voltage usually of some hundreds of volts.
The inverter turns this into AC (alternating current whose voltage looks like a sine wave) at mains voltage.
The AC side of the inverter is connected to your house wiring and the grid.
The AC from the inverter is exactly synchronised to the grid so that their voltages go up and down together and don't fight, but when the panels are generating, the AC made by the inverter is just a little higher voltage than the grid at your consumer unit. This ensures that it's the power from the inverter that feeds all your appliances and and anything left over is pushed out into the grid.
Electrical wiring has very low resistance so the voltage difference needed to push a few kW out into the grid isn't very much.

A household backup battery like any other, is a chemical device that is charged up with DC current and gives DC back when it discharges.
The majority of household storage batteries, including such as the tesla powerwall include a "battery controller" that :
1. Takes AC from the mains, rectifies it into DC and changes it's voltage to the right value to charge up the battery.
2. Takes DC from the battery and converts it to (nominally) 240V AC, synchronised with the mains, at just the right voltage to feed your appliances instead of (or in support of) the grid.

A storage battery with this sort of controller is said to be AC coupled because as a unit, it uses and gives back AC mains power. The use of DC is entirely within the battery and we don't need to be aware of that.
AC coupled battery storage can be added to any installation whether it has solar or not, and the brands of the battery storage and any solar inverters can be mixed and matched. They are independent modules. The battery unit is responsible itself for looking at current flows to and from the grid and may be programmed to charge according to time of day or (say) to absorb any surplus power that would otherwise go to the grid.

If you have solar PV and an AC coupled battery, the panels make DC power, the inverter turns it into AC mains power available to all at the consumer unit, the battery controller draws on this AC power, converts it to a different voltage of DC and charges the battery (chemical energy).

Do you notice the two functions of the battery controller 1 and 2 above ? Well 2 is pretty much the same function that the solar inverter is doing except the DC comes from the panels. In order to save duplication and reduce cost, the hybrid DC system essentially adds the functions of a battery charger into the inverter. The result is a single more sophisticated box instead of two separate ones.

Because the inverter now controls all the battery charging, the DC terminals of the battery are directly connected to the hybrid (inverter + battery controller). The hybrid inverter has added flexibility to take it's DC power from either the panels, the battery or both and turn it into 240VAC.

On the charge side, the hybrid box can take energy from the mains and convert it to charge the battery, but it can also take DC directly from the panels and use that at reduced voltage but without having to convert it from AC.

There is a small but helpful benefit here because each time you convert between AC and DC you lose a couple of percent of the energy. Hybrid inverters can use solar DC to charge the battery without going through AC. This reduces losses.

The down side - the battery and the inverter MUST be designed to work together. My GEN24 inverter is a hybrid type designed to control, to charge and to draw from the BYD battery.


Hope this is understandable. Hope it helps !

 

[pe2dave]

I told @pe2dave that I would answer his question about AC coupled vs hybrid DC battery storage in "a few days". Well it's been nearly 9 months so I had better have a go !


A storage battery with this sort of controller is said to be AC coupled because as a unit, it uses and gives back AC mains power. The use of DC is entirely within the battery and we don't need to be aware of that.


Hope this is understandable. Hope it helps !

Yes, thanks @Sideways - though I'd have called 'em AC charged batteries ;-)
The rest of the piece is interesting - makes solar that much more complex?

 

[Sideways]

We've been batteried up for nearly 3 weeks now.
Here are the January consumption and generation graphs.

What can we learn from these ?
1. At this time of year when solar doesn't make enough for our full daily needs, charging the battery has almost entirely taken priority over diverting excess electricity to heating hot water. That's OK because the electricity saving is much more valuable, but the "free" hot water is nice to have and I'm looking forward to getting it back in a few weeks.

2. You can see that export to grid has totally stopped. The battery soaks up everything we can generate at this time of year and on the one really good day so far, eddi the solar diverter gave us half a tank of water once the battery was completely full.

3. We find that we need about 25% of the battery capacity to take us through night. A full charge will carry us through 2 nights and a bad day in between. The battery is able to keep up with the oven and boiling a kettle or using the microwave, but cooking a full evening meal does use up another 10-20% capacity.

4. Overnight power use is between 250 and 500W. This seems a lot and has sent me looking for where it's all being wasted. The fridge freezer is actually pretty new and efficient. The surprises have been in the standby power of electricals. Wall wart power supplies, kitchen radio, PC etc really add up. The worst offenders are the TV (nearly as much power on standby as turned on), the paper shredder for heaven's sake, and even things like a toothbrush charger costs £8 a year to leave plugged in on 22p / unit. I'm putting some of these on timers to switch off and hence eliminate the idle current when we don't use them. I was again surprised to find an old mechanical timer uses just 4mA of current vs a digital timer's 14mA, vs 200mA for the TV on idle. 4mA x 24 hrs on the timer is a good trade for 200mA x 12 hrs saved in the TV.

As you can see below, the battery went in around 12th Jan. It immediately started soaking up all of our excess power but it isn't bright enough in January for us to be self sufficient and we were still drawing on the grid at some point most days.

These curves don't show battery state of charge, but that is all over the place. We made it to 100% just a couple of times. The sunshine and generation is visibly improving as the days go by and we can feel that we're past the winter low point.

 

[furnace]

£8 a year to run a toothbrush charger! Great statistic.
Maybe the 250-500W overnight isn't actually too bad in winter, since entropy allows that to be "useful" in reducing the heating load. Summer's a different story though.
Please continue providing the updates on your journey. It's really informative.
M