One family's solar story

[Dabop]

I think we can get 4 panels on each side for 12 total and with a largeish battery should be enough for for our fairly low usage except in the dead of winter and with the octopus flex tarrif we will buy at cheap rate to top up in winter. Long term it is certainly a wise move but we need to find the right system.

Ollie

I am offgrid these days (on a 'temporary' system at the shed that runs the shed and my caravan, while building the house) that supplies 100% of my power here- only 1.5kw of panels (3 250w facing north, and 3 250w facing west), charging the 20kwh lithium (LYP) battery bank that feeds the 12v lighting circuits, the 12v tv (inbuilt lol) in the van and the 12v water pumps for the sink and the shower in the van on-suite, there's also an rather elderly 8kw 12v inverter (I bought it back in 2014 or so, to fit to my ute lol)- this system allows me about 7-8kwh a day in spring and autumn, a fraction less in summer and in winter drops to about 5-6kwh per day... (the summer heat kills the panels outputs by anything up to 20-25% when the summer air temps are over 40C lol- just when running the aircon hits the hardest)

Helpful hints- avoid L/A (lead/acid- ie gel/flooded/AGM) batteries- go for the lithiums (LFP (LiFePO4) are ok, cost about twice as much as L/A but last MUCH longer and have more usable Ah of storage than L/A with the same 'label' Ah) LYP (LiFeYPO4) are slightly more expensive than LFP, but have a much wider temperate range than LFP- with L/A and LFP needing to taper off their charge rate when the battery temp goes higher than 40C, where the LYP can still charge at its full rate up above 60C and doesn't start to temperature taper until it hits a battery temp of 80C!!!! (important when we get temps over 40C for weeks on end here in summer) (they also work better than L/A or LFP in sub zero temps)

Batts- don't be fooled by the 'label' Amphours rating...
You won't get it!!!
L/A should really never be taken below 50% DOD (depth of discharge) on a regular basis, or their life expectancy will be very short indeed- at 50% DOD a L/A will have around 3-5 years life expectancy before needing replacement, where you reduce that to 25% DOD and you are looking about 5-7 years, at 10% DOD its 8-10 years- but that means you are only capable of using 10% of your storage capacity (eg a 100Ah battery, only using it as a '10Ah' one

Lithiums (dont use li-ion/LiPO) like LFP cost about twice as much per 'label Ah' as L/A, and LYP about 2.5 times what L/A does, but you can get 13-15 years at 80% DOD, and 18-20 years at 70% DOD- making them much better long term value that L/A or Li-ion ones

Use only MPPT charge controllers (and the higher the PV input voltage the better), mine are 150v PVmax rated for a battery bank between 12-48v- the battery bank voltage will set the total array wattage they can handle
(in my case, they are 60A each, at 12v they can handle 750w per controller, 1.5kw at 24v or 3k at 48v)
12v nominal x 60A means 13.8v charge voltage 13.8v x 60A = 828W, the manufacturer derates it by 10%, making its 'rated limit' 745.2w- they say 750w is good lol

 

[glenfield2]

It’s a fascinating thread but the amount of knowledge, time and research the OP put into it makes me, in some ways, even less willing to consider a system for our house. There are so many cowboy (or simply second rate) operators out there and even though I’m not entirely electrically illiterate (and we have solar panels on our canal boat) I’m not sure I would feel confident in having wisely spent maybe £20k.

 

[Sideways]

It’s a fascinating thread but the amount of knowledge, time and research the OP put into it makes me, in some ways, even less willing to consider a system for our house. There are so many cowboy (or simply second rate) operators out there and even though I’m not entirely electrically illiterate (and we have solar panels on our canal boat) I’m not sure I would feel confident in having wisely spent maybe £20k.

That's a very fair point. I have asked myself what advice I would offer to people without the electrical background.
First is that our array and battery are much bigger than the standard 4kW system. We have the roof space for it and always wanted to exploit that. Most people should be spending half the budget for a 12 panel setup.
Second is that I would always recommend buying reputable components. Maybe I should do a post on that specifically.
Third, I would search out a local ish installer who has been around for a while and built up some references. I would be reluctant to use one of the national companies who simply subcontract the installation of a standard kit of bits to random local tradesmen. Going local means you can get hold of them, they may be more invested in their reputation, and you have a better chance of getting a sysyem designed to suit you rather than a one fits all package.

With our 24 panel array we are almost self sufficient for 9 months of the year and make a real surplus in summer.
With 12 panels and an 8 to 10kWh battery you might be close to self sufficient for more like 7 months and not be wondering how to burn off the excess in midsummer. A battery of that size will take you through the night and probably power your oven to roast a family dinner. It won't carry you through two days without sunshine. If you are interested in using an Octopus flex tarrif where you can charge the battery cheaply overnight to power you through the day regardless of sunshine, then perhaps 12kWh of storage would be nice.

HTH

 

[mc281]

Our install was pricey. I gave some numbers earlier. About £24k all up I think but for a system that is 2-3x the capacity of a typical 4kW system. There was about £18k of materials cost in that. I know as I chose or approved all the components that went into it. We bought premium parts with long warranties and top quality build then Covid supply shortages meant no chance of discounts on the parts.
24 panels on one single roof face plus the wiring up basically took 2 guys and an apprentice a solid week. A skilled sparky came in extra for a full day to swap the consumer unit and later half a man day to build and integrate the battery stack.

Re the optimisers, waste of money if the panels aren't shaded.
Re the two string inverter, this is pretty standard. Three very different roof faces would need 3 strings and that might push you to two inverters. 2 inverters together imposes some limitations compared to using just one but a good basic 3kW inverter isn't more than £1,000

Take a good look at the components proposed by your different bidders. That's where a lot of them will be cutting costs. They will use cheap brands and limited capacities.

Why isn't there an independent test installation paid for by prospective PV users making say a £50 payment to obtain FACTUAL PROVEN INDEPENDENT INFORMATION about every aspect of PV installation, home generation, storage and TRUE INDEPENDENT COST Benefiy, CARBON Deficit, AND FULL EVERYTHING Analysis for every situation?
If PV is as good as it is touted to be by the powers that be, the government or the generation companies who are saving so much on generation costs could give back £100 to everyone who supports the independent testbed and decides to have PV installed.

 

[Sideways]

I'd want a lot more than 50 quid to do that for you

There's a scheme a bit like that for properties - the EPC - costs £100 and you get back a piece of paper based largely on tick boxes. Government mandated schemes like this usually descend to a low level because doing it properly requires detailed knowledge and that has to be paid for.

To be fair, one really good thing about the MCS (Microgeneration Certification Scheme) is that all the solar companies registered for it have to do their generation forecasts the same way, using the same numbers for sunshine in any given place. This should allow a fair comparison.

Their document is public so you can download it and do the maths for any solar array yourself. If you are technically minded it's a good read (quite detailed but presented very accessibly) and by seeing how the solar companies are supposed to do it, you'll recognise a properly prepared quotation when you get one.

Long term independent tests of equipment and especially battery performance were of great interest to me and a scheme like you describe would be brilliant. There was a voluntary one in Australia for batteries that I've referenced in post #49 and that had a big influence on my buying decision. It's the only rigorous, independent source I found that made it's results public. Huge kudos to the Aussies for setting that up and running it for a few years.

There is a rating scheme for PV panel manufacturers based on quality statistics. That was useful too. I'll dig out some links and share that. Bottom line, there are several big panel manufacturers who make reliable product with perhaps not a lot between them. There are a small number of cutting edge panels that offer longer life with higher performance but you pay a premium for them. There's a mass of no name stuff at lower cost but you have no way to know if it will be reliable or not.
Panel tech is evolving continually and individual designs don't stay in production for many years. New panels of obsolete designs fetch premium money because if you have a single panel fail in an array after 10 or 15 years, you would want to replace that panel with the same thing. That means manufacturers have to hold stock for years based on volumes sold and statistical failure rates. Just like spares for our tools and machinery are always expensive !

 

[Spectric]

Looking at the fact you have surplus and cannot supply the grid or store it then for some people maybe the answer is to have some joint venture where if you have a neighbour with an interest then sharing the cost and power would resolve the issues, I know there are a lot of buts and potential issues but it is one way to look. There are already community systems, albeit small that have industrial ground source systems that supply upto I believe 300 properties and was shown on Tv, not sure if it was countryfile.

 

[Lard]

Could someone either confirm or refute my thoughts on battery storage.
We had solar fitted about 3yrs back and it's on track to payback around the 9yr mark.
I, like a lot of people, would love to have battery storage fitted but (and here's my point) for approx 3-4 months of the year our 4.2kw system struggles to produce anything over 3kw per day (compared to a good summer where it max's out at around 27kw per day).

I think I'm correct in stating that if a 5kw battery (smallest available?) doesn't get a full charge/discharge cycle every day, it reduces the lifespan of the battery, which is a max of, supposedly, 10yrs?.......now, I've done some very quick calcs and concluded that even if I could get 10yrs out of a battery, the cost of installation versus savings would result in financial break even at around 10yrs.....meaning that just as you break even you then have to replace the battery.

Conclusion - cheaper to pay for electricity from the grid instead of purchasing a battery.

I'd love to be proven THAT wrong in order for me to have a rethink.

 

[HamsterJam]

We have a 13 panel system with optimisers which was installed a few years ago by the previous owners of the house. It has components with long manufacturer warranties (LG panels and Solar Edge optimisers/inverter), configured as a single string.

Unfortunately we have a fault where one of the optimisers isn’t reporting data to the inverter. I also suspect that panel isn’t generating.
I tried contacting the original installer but they went out of business in 2017 and no-one else I tried is interested in fixing a fault (although they are more than happy to fit a replacement system )

It also appears the manufacturers won’t deal directly with consumers so I have little hope of anything being replaced under warranty.

So far I have identified which panel/optimiser pair is faulty (by shading each one in turn) so next job is to go up on the roof and check the wiring, panel output voltage, etc. I will also attempt to swap the optimiser with one on an adjacent panel to see if the fault transfers.

Does anyone have any tips for fault finding on solar systems please?

 

[John Brown]

Could someone either confirm or refute my thoughts on battery storage.
We had solar fitted about 3yrs back and it's on track to payback around the 9yr mark.
I, like a lot of people, would love to have battery storage fitted but (and here's my point) for approx 3-4 months of the year our 4.2kw system struggles to produce anything over 3kw per day (compared to a good summer where it max's out at around 27kw per day).

I think I'm correct in stating that if a 5kw battery (smallest available?) doesn't get a full charge/discharge cycle every day, it reduces the lifespan of the battery, which is a max of, supposedly, 10yrs?.......now, I've done some very quick calcs and concluded that even if I could get 10yrs out of a battery, the cost of installation versus savings would result in financial break even at around 10yrs.....meaning that just as you break even you then have to replace the battery.

Conclusion - cheaper to pay for electricity from the grid instead of purchasing a battery.

I'd love to be proven THAT wrong in order for me to have a rethink.

I assume that you are using kW when you mean kWh. In the context of your post it is unlikely to be misunderstood, but thought I'd point it out anyway, in what I hope is a helpful way.

 

[Sideways]

No. You're misinformed. Unless I am !
Batteries lose capacity as the number of cycles they've been through rack up.
They also have a calendar lifespan even if you don't cycle them.
Guessing where these will work out for a given battery is the million dollar (or maybe £10k) gamble.
From my reading, the calendar life of a good battery may stretch to 15-20 years tops.

You may want to replace a battery when it has dropped to 80% or even 60% of original capacity.
This is not so important with a home battery but would affect the range of an electric car so be a much bigger deal.
Home batteries should be a technology like LiFePO4 which is much heavier / low density than the batteries fitted to cars but has a far longer life.

It is common for batteries to be warranted for a set number of years AND either a total number of megawatts or number of full charge discharge cycles which is another way of saying the same thing.

For the numbers, it may be prudent to assume you will need to replace your battery after say 10 years. Like you, on that basis I think this makes solar storage look like a break even deal with using the grid and it's why I described our system as a prepayment plan rather than an investment opportunity. Nonetheless by buying what I hope will prove to be a quality battery and bigger than our essential needs, i am hoping that it will still be working and still have enough capacity for our needs for more like 15 years which will be a significant upside for us.
The solar panels themselves I'm hoping will last 25yr plus and ideally see my wife out at her target of 98, long after I'm dead

Two half charge discharge cycles are basically equivalent to one full cycle. Lithiums do not deteriorate because you partially cycle them. If anything, this stresses them less.

Might be worth looking again at your numbers, but it is a gamble to assume any more than the warranted capacity of a battery.
Rather like best before dates at the supermarket, the manufacturers are going to err on the safe side. It would be waste to toss a battery the day after it's warranty ends. You might get quite a few more years.
On the other hand, some people will fully charge a battery every day on a cheap car or overnight tarriff and export the whole lot at peak time to earn money from the arbitrage. You'll wear your battery out faster that way but depending on the prices of the battery and the profit you can make on a kW, you might make money.

As "vehicle to grid" takes off, there will be people doing this using their electric car batteries to make a few quid. EVs have much more capacity than a household battery so more money making potential. But EV batteries have shorter lives than home storage batteries and I believe V2G will kill them quickly. If you lease your EV on a 2 year contract you probably don't need to care but I for one wouldn't want to buy your 2yr old EV with a worn out battery in it. Watch out for lease contracts prohibiting use for V2G.

 

[Spectric]

Batteries have always been an issue, having worked on large UPS systems it was always a battery that caused a problem and no mater how well or regulary you tested the system, the point of failure always seemed to be when most needed. The charger has a large part to play, you want a super smooth output and a good multistage charger. Here conversion from single phase Ac to Dc is not as good as three phase to Dc and for a much better output you can use a three phase transformer with both a delta and a wye secondary which gives two Ac outputs with a phase difference so easier to rectify.

 

[Sideways]

Does anyone have any tips for fault finding on solar systems please?

Your panels are probably making 40v or more each. In a string of 13 that is over 500V DC.
The DC isn't referenced to ground so odds are you can touch one of those DC conductors without harm but touch both live terminals of the full string at once and they may be recovering your corpse from a tree down the street ! Just pay attention as you work so you only handle one of + and - at a time.

On a more positive note, individual solar edge optimisers aren't that expensive.
I'm not a great fan of Midsummer as they messed me about twice, but they are very transparent about retail pricing and will sell to anyone.
https://midsummerwholesale.co.uk/buy/SolarEdge
If you are happy to do the fault finding you describe, you could most likely swap out an optimiser yourself at a low enough cost that it isn't worth chasing down the warranty.

I haven't done this but I'd contact the MCS scheme organisers and ask their advice about how to get warranty work done following the insolvency of an installer who would have been registered with them.

Like you, I have found my inverter manufacturer reluctant to deal with end users (even when you know more than the average installer) and it annoys me big time. If I were chasing a significant warranty issue with them, I'd be writing strongly worded letters maybe aided by citizens advice, and escalating. But for one optimiser it isn't worth it. If the panel has gone too, then reconsider. Also look to see if your package includes an insurance backed warranty as you would need this if installers poor workmanship left you needing roof repairs years after the install. This type of cover might last for 7 (?) years and be another way in.

 

[Terry - Somerset]

I have read this thread with interest which inevitably some thoughts:

Space heating
Electricity accounts for ~50% of energy costs. Gas consumption is ~3-4 times as high but ~25% of the unit cost. Using electricity for space heating or hot water mostly only works with a heat pump with a CoP of 3-4.

Sizing a system to provide space heating used mainly in the winter months would simply drive up overcapacity for the remainder if the year.

Economics of batteries
Batteries storing cheap rate night time supply for use during the day relies upon a differential day and night rates for many years, and the cost and effective working life of batteries.

Battery prices are reducing - in respect of EVs there are reports of a cost per kwh of ~£100. However the cost of a (say)10kwh battery seems to be currently in the order of £5-8k. Assume - 10kwh battery cost £5k, life 10 years:

cost per kwh stored £500pa/365/10 = 14p kwh.

With probable growth of V2G it seems unlikely day/night rate differentials will be maintained for the next decade. However the cost of domestic battery storage may have some way to fall!

PV installation
I see advertisements for a 4kw installation at £6-8k. Depending on orientation, location etc this may generate ~3500kwh pa - roughly equal to my electricity consumption. Assume - Cost £7k, life 20 years, average maintenance (??) £200pa, output 3500kwh:

cost per kwh £7k/20+200/3500=16p kwh.

If only (say) 30% of home generation were used and the balance dumped the effective cost per kwh would be ~53p kwh. if all stored (14p kwh) and used ~30p kwh, if the surplus returned to the grid at (say) 5p kwh the cost per kwh used would be ~40p.

PV + battery is close to competitive with current grid costs of 27p per unit but hardly compelling - of course future grid price changes are wholly uncertain.

Selling surplus capacity to a neighbour
I am not sure of any legal or technical complexities - but this could be a win-win solution - with a grid price of (say) 27p a neighbour may be willing to take surplus generation at (say) 20p.

Conclusions
The above uses very crude cost estimates without regard for system quality and design - it is not intended as a definitive analysis. It is not a sophisticated financial appraisal with risks evaluated against a discounted cash flow model!

Based on this the installation of PV and batteries may make little financial sense although it depends assumptions made about the future trajectory of grid energy costs, battery and other system costs and effective working lives.

However I would add two principle caveats to this simplistic appraisal:

• satisfaction gained through knowing that one is contributing to a better environment
• the analysis is likely to change radically with ownership of an EV acting as either V2G storage and/or charged from home PV generation

 

[Sideways]

My 16.6kWh BYD battery is warranted for 51 Megawatt hours throughput.
(Screenshot of the warranty below)
That's 51,000 kWh
Assuming some loss of capacity over time, the average capacity of the battery pack is maybe 14kWh on average across it's whole life.
51,000 kWh / 14 kWh = 3,643 full cycles, or one full cycle a day for 10 years.

In practice, we don't fully cycle the battery daily even in winter, so the battery is likely to keep going until age kills it, not #cycles.
In money terms, it cost £10k, so £10,000/51,000kWh = 19.6p/kWh if it lasts only the warrantied throughput.
Using it to store free solar electricity for our own use instead of buying from the grid works out. We have a 10 year fixed price deal in effect at a decent rate.
Using it to save just 10p a kWh by charging with mildly discounted overnight electricity and then incurring 19.6p/ kWh wear and tear on the battery doesn't.
If we had an EV as well and access to a MUCH cheaper overnight electricity tarriff, then charging from the grid in winter might.

The real key is to get good batteries significantly cheaper than £3k per 5kWh.
I couldn't when I was buying but there are new options arriving all the time.

 

[HamsterJam]

Your panels are probably making 40v or more each. In a string of 13 that is over 500V DC.

If you are happy to do the fault finding you describe, you could most likely swap out an optimiser yourself at a low enough cost that it isn't worth chasing down the warranty.

I haven't done this but I'd contact the MCS scheme organisers and ask their advice about how to get warranty work done following the insolvency of an installer who would have been registered with them.

Thanks Sideways.

Aware of the voltages and currents involved so won’t be going on the roof with the system energised. The optimisers get a signal from the inverter to turn on their output. Until then they provide 1v. A 13 panel string should therefore output 13v offline. I measured 12.5v which wasn’t very helpful for fault diagnosis.

Thanks for the supplier detail and I hadn’t considered contacting MCS.
Given the cost of the parts, I’ll proceed to find the fault and if low cost enough just repair it and ignore the warranties.

 

[Dabop]

There are some rather dodgy numbers being thrown around- LFP/LYP cells are rated for 5000 cycles at 80% DOD, and 7000 plus cycles at 70% DOD (like all battery packs, the shallower the DOD is, the more cycles you get out of them- a 50% DOD on LFP or LYP cells will be out past 10000 cycles... (with a single 'charge/discharge cycle for offgrid use being one day and night, my battery pack will last 13-15 years at 80% DOD, and 18-20 years plus at 70% DOD

Li-ion cells (ie the ones found in Tesla powerwalls and some of their cars) have a higher charge density (smaller cells for the same physical size) as LFP/LYP, all are far superior to L/A by far, but li-ion is considerably poorer than either LFP or LYP cells- Tesla uses them because they store the most- but at the expense of long service life (indeed Tesla are now using battery packs made by BYD in China for their 'base model' range, using LFP technology... The Powerwall li-ion cells have a lifespan of 10-12 years before they fall to 80% of their new capacity, where the LFP/LYP cells are about double that life expectancy... a BIG difference...

The company that makes my batteries have been selling them in Australia since 2008, so they aren't exactly a 'new kid on the block' and have the data to back up their lifespan claims...
These are the specs for my battery bank (16 of these 400Ah cells, currently running at 12v nominal in 4S4P configuration, eventually when the house is finished, reconfigured as a 48v 16S instead, nominal 20kwh of storage...)

That gives me a battery bank with about twice the lifespan of a Tesla Powerwall, at about 2/3 the price and higher actual storage capacity....

During setup...

 

[Sideways]

@Dabop That's great information. Thanks !

My battery charge and discharge is completely managed by the Fronius and BYD kit which sets max and minimum charging with a default range of 100% - 5% state of charge.
The manuals say that you can use this without concerns.
In fact, without doing a load test I don't know if they are including any margin in the actual state of charge / battery capacity. All I know is that when I draw say 10kWh, of power, the state of charge does drop by 60% give or take, which is what I would expect. This is also the typical maximum depth of discharge we would do in one day.

Now, in the software config, I can if I want, set different limits for max and min state of charge.
Given that the UK has cold enough winters and can go a few days with little sun, i decided to set a minimum state of charge of 15% rather than the default 5%. If the battery gets too low, the charge controller will draw from the grid automatically to protect it but with my 15% minimum there is no risk that it will ever fall dangerously low.
I left the max limit for SOC at 100% but I'm considering dropping that to 95% at least during the summers where we top out the battery every day and rarely draw it down below 50%. I'd read something about cells physically expanding as they charge but especially when you try to force the maximum possible charge into them and that cells last longer if you don't do that.

As we have more capacity than we need most of the time, from your own understanding, do you think that 15% to 95% charge limits would benefit the battery life or am I being unnecessarily careful ?

 

[Dabop]

I suspect they must be using that 80% DOD as '100% of rated capacity- actually hitting a 95% DOD (5% left) of actual capacity is dangerously overdischarging them... and would reduce their lifespan significantly....

(it would be nice if they all standardised their measurements lol)

BYD use LiFePO4 ie LFP cells (they call them 'blade cells') which have some surprisingly low values- their max charge voltage is only 3.65v per cell, where my LYPs can handle up to 4v per cell on charge- I can't find any specs on BYDs charge voltages in their charge controller/BMS (rather than their % figures) so its impossible to tell exactly what they are classifying as '100%' I would 'assume' it is the max voltage per cell ie 3.65V per cell, so their 48v nominal pack would be 16 series cells and 100% would be 58.4V- but that is only an assumption, their data simply doesn't say unfortunately

Running them at less than 100% max charge voltage doesn't do them any harm (unlike L/A), in fact although mine can handle up to 64v for a 16S '48v' pack, I keep it well under at 13.8-14.2v for my LYPs ie 3.55v per cell (still running at '12v' to power the van ie only a 4S arrangement) as most imbalances occur at both ends of the charge cycle (near fully charged and fully discharged)- staying away from those will avoid the BMS having to 'charge shuffle'...

That 5% is worrying low for a LFP if thats the actual DOD (95% DOD) as most manufacturers don't spec under 80%!!!- but if it is 5% of that 80%, then thats a different story- it would be an actual 76% DOD max recommended discharge, which would give you about 6000 charge cycles before hitting that '80% of new capacity' figure... ie 16-18 years of effective service life... If this is the case, then setting your lower limit to 15% like you have would be an effective DOD of 68% DOD, so you could expect 7000 cycles plus- probably closer to 8000 cycles or 21-22 years at least!!!

BYD do make very good batteries (they are bigger than Tesla, and have been making their own LFP cells for use in their EV cars since 2009 and EV trucks and buses since 2012, so they are again a well established company with lots of experience- bu their data specs are hard to find (mostly because until recently, all their production was used 'in house' in their EVs....ie no one apart from their own service departments could even get them lol)

 

[Sideways]

Thanks for searching
I do suspect that the 5% bottom is not a true 5%, so I won't be worrying about this but since I can, I will also go ahead and drop the upper limit to 95%.
I have noticed just from watching the way the SOC % changes on the app that the charge rate must slow down near 100%. The controller can hammer 5 and 6kW into the battery no problem then sit at 98 and 99% for quite a while squeezing the last bit of charge in.

Cheers

 

[Dabop]

Where that 'slowdown' is in the charging- that is the bulk charge is over, and the BMS is 'charge shuffling' to equalise the cell voltages- this is the bit where it is best to avoid being....