Before adding a comment I'd recommend reading this community post where I've added a bit of extra detail to better illustrate how I view the payback of the extra battery: kzfaq.infoUgkx3IwGg11F0Q8f0WOVl_RVqag2jGQS2UI3

Just had a second Powerwall fitted and have been surprised by how much more flexibility it gives us which means we run the dishwasher and washing machine when we want, its much easier to use all the electricity we generate and the ability to charge the car directly from the battery is very useful (charge and discharge is now 10kWh), when we get a second Air-Air heat pump we can run it on cheep electricity. As I don't sell back to the grid (I'm on the last of the FIT) I don't have to worry about excessive cycling. I have found focusing just on the financials can blind you to other benefits.

Great analysis Tim, thanks.

Thank you! The conclusion has not changed, feels like battery costs need to come down to make this viable……

Top video Tim. Thanks. Best regards Martin

Did I miss the calculations for conversion losses? What about leaving an extra buffer? You won't want to force discharge all of the battery every day. A higher power inverter such as a Sunsynk could discharge at a much quicker rate than the GivEnergy does. This video reinforces my view that home storage batteries are not a no brainer. Calculations are always needed.

Nice video Tim - thanks. The interesting bit that I picked up in all this was your understandable reluctance to manually insert into your lifestyle any daily force discharging to the grid. We installed 2 x Powerwall2 late last year and one of the beauties of that system is that if you put it in “reduce cost” mode, it does all of those calcs itself and effectively ensures that by the start of the cheap/recharge period, you have dumped any extraneous power to the grid to pickup the arbitrage. Even better, even though we are on Outgoing Octopus Fixed, so exporting is always 15p, I’ve configured the Tesla gateway to think that we make more money by exporting between 5pm and 7pm, so it keeps the battery as full as it can up until 5pm, and then throws everything it’s got at the grid just at the peak usage period. And all of this with no manual input by me. It occasionally misjudges and we run out of the battery before the cheap period starts, but not often and not by much. So there are some advantages to the Powerwall setup.

Thanks Tim, I did find it useful to watch you work through these numbers. Wow, you use a lot of power. If we ever meet up we must do some U-Value calculations something seems wrong somewhere!? I agree with your conclusions, when you think we don't get 100% sunshine and installation costs. Expensive for benefit as there are other items you didn't take into account as I am sure you will know. Shame really, I was hoping I could buy that 5.2 Kwh battery from you AS I am doing similar calculations for my system..🙂

Thanks for this! It saves me working it out.

Great analysis. 14kwh is ample - my analysis of my heat pump house’s usage was that 9.5kwh battery would give me 80% of the financial value of a 44kwh battery (which would cover all import). Still deciding whether to get an AIO though.

This is great, thank you. I've been going through the same process recently; we have 9.5kwh and over winter it pretty much always runs out at some point between lunchtime on the coldest days or mid evening most days. I concluded that even though an evening of peak rate electricity is often several times more than the rest of the day's offpeak, it's still cheaper than investing in a new battery. Buuuuuut... I'm still leaning towards getting another 9.5kwh (or maybe just a 5.2) before next winter anyway, because it's not just the financial value, there is value in the satisfaction of being able to make better use of our soar generation all year round, and avoiding using peak time grid electricity when it's at its dirtiest. This time of year annoys me because we export lots during the day when the battery is full, but it still runs out before the sun comes up the day after if we leave the heating on. Plus maybe sharing the load between two batteries will make them last a bit longer...?

Great video (as usual) I have 2x reasons to increase the battery capacity. 1) I have a Solax system and can force discharge at 5kWh 2) and the biggest for me…. I would like to retire in 8 years or so. I currently earn a nice income and whilst I could invest the surplus cash, I have chosen to improve my solar and battery capability and eventually retire without a huge gas/ electric bill each month. My monthly bill was £278 and it’s now £80. I don’t think that £14,000 sat in the bank would have made me £198 per month. So when people ask when my ROI will start, I feel that my answer should be “instantly” or “last year”

Another consideration would be to look into the practicality of whether additional battery can actually be charged during the cheap tariff period based on inverter and battery max charging rates

Worth pointing out that force discharging your GE battery at the end of the day is straightforward. I've got it set up in the app to export anything left in the battery at the start of the off peak period. So I make a little bit of money with whatever I didn't use. All automated so no hassle or manual input required

That automation is really not difficult. Happy to help you, show you my automations which work with IO

It's worth mentioning if you have a 3 phase connection you can discharge 3.6KW per phase so 10.8KW in total

Hi Tim, So many variables to consider. Presumably the changes in the overall cost of electricity over time is less important than the difference between import and export? Not only a second hand battery market to consider but improvements in battery technology to keep an eye on. Also vehicle to grid technology can be an option in the future.

Interesting analysis and video. I’ve done exactly the same thing as you’ve described but with Tesla powerwalls and solar panels. However we have a heat pump and 2 x EVs to charge. In my case my calculations worked out in favour of getting the additional storage , but it is very much a case by case on your personal usage and circumstances. The one thing you didn’t touch on though that’s probably worth thinking about is the potential future rise in grid import costs per kWh in the future. You might find in years 8 onwards for example the cost is much higher which means your payback is quicker than originally estimated.

We have 4kw solar array running a 10kw system with 31.5kWh of storage cost us about £10k 2 years ago. I plan to run the batteries at 80% of thier capacity durring good solar production, and 100% durring the winter months to help prolong the life of the batteries.

Glad to have this analysis Tim but the cat is only deductible if installed by an installer and not reclaimable by individuals- I tried to buy an extra battery and I couldn’t find anyone that would sell one vat free.

I did consider however, the route I now think is using my cars massive 65 kWh battery to backfeed back in to the house, it can already feed a 13 Amp socket from the charge socket so a suitable EV charger and car and we are looking at 7Kw to the consumers unit

My current thinking is that what would make most difference to me is going to an AIO, mostly for the additional power capability. Don’t think they’re yet usable alongside the “normal” AC coupled inverters though.

I changed from 10kWh to 15kWh hours battery last summer as a number of times through the previous winter on a standard tariff I was running out. But then I got OI Go and in the last winter I very really dropped below 50% and that was only when I took part in a saving session, so now I'm wishing I had held fire and stuck with the 10kWh as its not working very hard at all. The only thing it gives me is that I don't have worry too much about pushing dishwasher and tumble dryer usage into the off peak time slots as there is always enough battery power. I guess the 15kWh would be useful if I add a heat pump but at the moment I can't see how I can make that pay for itself.

I jumped in a few months ago and added a 2nd 9kw battery. It’s increased the payback but I think the return on investment for me was the most important factor - 40yrs in financial services!! Love your monthly updates as I have 12 south facing panels and hence with your EW array we are very similar. Sadly I’m having a battle with Give as 2 days ago the system mysteriously showed minus 850kwh generation to home. This has completely mucked up cumulative data and makes a nonsense of the break even report. They don’t seem to be interested at all - I hope it never happens to you !😢

Would have been good to give us a link to your initial calculations before you got a battery at all for comparison as you probably made a video of that as well?

Very interesting - I have 2 9.5 batteries with a 5kw inverter. The issue is the batteries will only output a max of 2.6 kw. I would like to have 2 inverters put a battery on each so that when I need higher output I have it. That solution is not currently available. So in conclusion I have the battery source but can’t use it to its potential. That needs taking into account when adding batteries.

I have 3x 3.6k kWh batteries. I am thinking about getting more if our heat pump passes planning permission. I believe my batteries will be flat before the Agile peak time if I don’t have some more.

Hi Tim. You've done a good example of the current costings however you've not taken into account that over 10 years the cost for electricity would have increased and therefore even at today's cost of the battery and/or the price of batteries going down that in the 10 years there would be a saving that would increase year on year

31kwh here has only just been enough for us over Winter without a Heat Pump. At least twice daily usage I'd recommend. Just bought a 3rd 15.5kwh battery to give us 46kwh battery for £1950 and they are truly awesome. 180amps charge/ discharge per pack. April until September last yr - 100kwh from Grid. £25 for 6 months. 5kw output with battery only, 6kw output with some Solar available. Solis 6kw Inverter - £1150 3x 15.5kwh batteries - £5850. Now enough to store 70% of a full car charge. At 0.18C rate, they'll last 26+ years and since first installed 24th April last year, saved over £1600 on our electricity and gas bills.

Tim Great video on battery capacity but you have not considered buying an All in One 13.5 KWH battery rather than a 9.5 KWH battery as this would allow you to export more and also ensure you also don’t import so much power when your system is at capacity, although appreciate this final factor may not add much saving.

Yes, it seems as if you have a reasonable amount of storage. There are not too many days where your winter electricity uses peak energy, and your new heat pump running style certainly looks effective. Time to enjoy the garden, or else improve the insulation of your dot-and-dab walls? 🙂

Are you accounting for the losses from charging the battery and then converting DC to AC?

If you have a tame electrician who is able to open a trade account with Midsummer, the can benefit from a fairly substantial trade discount. That might make a difference to your cost / benefit equation.

Hi Tim Have Octopus switched your tariff to IOF?

Pretty similar to my guess that it is just not worth it to oversize the battery. Not to mention can you actually get enough electricity out of the Inverter ?

Is that chart showing an average import cost of 26.3p (£157.96/599) We have a similar setup with A2A, Solar, 13.5kWh Batteries but no EV and our import price averages at 21p on Octopus Cosy.

If you are adding an additional inverter, there is also another consideration/cost, which is you would need to go back to the DNO for approval for the additional generation. So not only is it an additional cost, but you may be restricted (G100) and wouldn't benefit from the whole capacity.

interesting and useful, as always as others have said, your 10 year projections did not have any inflation acknowledging there are quite a few variables and assumptions at play; energy provider tariffs, peak/off peak, arbitrage amounts, etc. Any of these can change at any time, altering the cost benefit analysis. having said that, I think relatively long term inflation (3% / year?) would be worth incorporating if your fans are considering their first solar + storage installation, they might buy larger battery + inverter up front if it means their insulated against rising prices for 12 years. the perception of energy security that solar + storage offers is also hard to beat; it isn't cheap, but it's reassuring in these turbulent times re second hand batteries; could you buy an old EV and use V2G? The car remains on your drive, now relegated to a stationary battery? In the early days of EVs, Nissan and Mercedes planned for this; they knew EV batteries would degrade; they might not be suitable for EVs, but would continue to have sufficient power and capacity to be repurposed as home batteries. Alas, that plan never materialised. If it had, things might have been very different... would you consider another thought experiment...? could you go *entirely* off grid? one of the highest efficiency and affordable panels are AIKO's n type all back contact (ABC) panels, coming in at nearly 24% efficiency. Another is aeg-solar.com/solar-modules/abc-back-contact-n-type/ if you've watched kzfaq.info/get/bejne/sLajZdOKmtmwm2Q.html you'll see that peak rating is more-or-less irrelevant in the UK. at the time of writing, these two panels will probably generate the most in the UK's less sunny and overcast conditions the glass-glass editions are warranted for 30 years if your *entire* roof was covered with these panels - "front" and "back", edge-to-edge, and you had something like Tesla Powerwall 3 [LFP battery (= long life); hybrid inverter (=eliminate solar DC - inverter AC - inverter DC - battery DC conversion losses, so more power), how much of the year could you go off grid? You might even have two Tesla Powerwall 3 batteries; that would give you 27KWh capacity but also redundancy [now that you're off grid] LFP batteries probably offer 20 year operational life [though warranted for less] GivEnergy now warrant their inverters for 12 years. Tesla have probably learnt a thing or two installing and monitoring 500,000 Powerwalls; should we be optimistic and say Tesla Powerwall 3 - including inverters - should operate for 20 years? National Grid need to invest many, many billions on restructuring the grid, and they're raising the standing charge - SUBSTANTIALLY - to fund it. It could rise quite a bit over 20 years, if the last 2-3 years is an indicator so, the cost benefit analysis might consider the investment against remaining connected over a 20 year lifespan The rising standing charge, increasing efficiency and effectiveness of solar and the projected fall in LFP battery prices (cleantechnica.com/2024/02/26/catl-byd-to-slash-battery-prices-by-50-in-2024-boom-evs-win/) mean going entirely off grid sounds more achievable. you might not be able to go off grid for the entire year; you might go off grid March-October, then reconnect for the four months with short, overcast days. Who knows, if, in 10 years, affordable, practical solar panels hit 30%-35% efficiency, you might be able to replace the panels you already have, and go off grid for the entire year In 2024, perhaps just a thought experiment; but as - technology becomes more efficient - technology lasts longer - technology capital costs fall - but labour costs rise - grid standing charge just keeps rising, ...going entirely off grid becomes worthy of consideration

17:44 Did you consider getting a 9.5kWh and selling the 5.2 (effectively you create the second hand market) to offset the cost and then switching to IO and giving you an extra two hours to charge and discharge?

Of course battery prices and technology are likely to change over the coming years. Your price model has possibly one issue, cost of electricity import/export this will change but by how much and will the ratios between import and export; peak and off peak change?

Tim it might be worth actually getting some quotes, I copied your calculation and got some quotes, For supply and fit: 8.2kWh Battery for £2550+VAT@0% 9.5kWh Battery for £2890+VAT@0%

Maybe I’ve missed something when watching, but in the summer wouldn’t a lot of your battery charge be “free” from your solar? So the payback would be higher?

I've got a 5KWh battery and have wondered about getting a second especially now VAT is removed. But my spreadsheet says I'd only have saved about 42p per day so far on average. So it's nowhere near worth it given I can't force export and don't have power cut backup. But things will probably change soon. We might be getting a heat pump, if we get an EV the arbitrage (good word that) between charge cost and peak saving will grow. I did find a seller of Huawei batteries in Germany on eBay. But post Brexit would mean importing would be a huge hassle. Shame as they're about £500 cheaper than I found in the UK.

Hi Tim, Some great calculations there but I believe there is a fundamental flaw in your assumptions if I may be so bold depending which way you look at it - but don't worry it's good news. It comes down to the assumed "payback period" and annual saving. You may have already made a calculation about the whole system cost of PV plus battery and heat pump and how much that saves. I don't know what your figures are but here's mine as an example. The whole system cost around £20k and saves around £3.5k per year compared to my previous combined gas and electricity consumption. I know that because that's what it did cost and I'm still positive in the Octopus account after banking the summer surplus on Flux and using cheap electricity Intelligent Go imported over winter i.e. I'm breaking even over the whole year so zero bills overall. Therefore the whole old system cost, is now a new system saving every year. 20k/3.5k = 5.7 years payback time. Now, if I do the same calculation as you've just done on the potential of adding some extra battery capacity (say double) to avoid peak tariff import in December/January etc then I might save say £300 per year and divided into the extra battery cost gives a payback period of about 25 years so just not worth it. ❕BUT hold on ✋❕ That assumption completely ignores the fact that my system is paying me £3,500 per year which would pay for said battery in two years not 25. Put another way if I removed my existing battery and then considered adding it back in should that same battery have a payback of 2, 5.7 or 25 years? 5.7 years of course because I know that already from the calculations of the whole system. You should base the calculation on the whole system not one element in isolation. It might be more obvious if you think of adding or subtracting just 1kWh of battery capacity. The total payback time is still going to be close to the system 5.7 years for that 1kWh either way. I hope that's encouraging😉👍

The mathematical formula for ideal capacity is n+5kWh….where n= what you have now 👍

Your figured and conclusions came out more or less the same as mine, there isn't the economic payback right now, but maybe in 5 years lithium batteries will be cheaper or sodium will be around, or they'll have cracked residential flow batteries. It's also hard to predict the price of residential electric in 10 years

Calculating round-trip inefficiencies through the inverter and battery is a bit tricky. Personally I think that the battery industry has reached a stage where the government should be asking battery and inverter manufacturers and suppliers to state expected losses, with a formal test. I'd bet there is a range of efficiencies in the market, and it would surely encourage manufacturers to strive for higher efficiency, and enable consumers to make a more informed choice.

I know people on Octopus that during the saving sessions saved over £20 a session with force export

One big omission is taking future energy cost into account. They never go down only up. in five years time they could be doubled up. If it kinda make sense now the rewards will come later.

Mid-Summer will charge you VAT - you will only be VAT free if you buy through a company and they WILL need to make a profit - in essence your figures do not that that into consideration ~ just saying

You can get 'server rack' batteries significantly cheaper from brands like Pylontech or Dyness, but then you might not be able to access Intelligent Flux.

Good content thanks. I suppose it doesn’t work if you look at it with just money and payback in mind. Bear in mind the trend for electricity cost is always going up not to mention the carbon cost. There will be a carbon tax soon no doubt. Great video though thanks.

Your calculations don't take into account that the 4 hr cheap period will only let you charge 4hr x 3.6kW = 14.4 kWh. You currently have 14.4kWh of battery storage so you cannot charge any more batteries up. If you moved to Eon next drive you would get 7 hrs of charge time @ £0.095/kWh and could charge up 25.2kWh of batteries

Good explanation. I have looked at increasing our battery capacity. Currently have two SolarEdge 10Kw batteries and a 6Kw inverter. Was thinking of adding another 10Kw battery but our 6Kw inverter would not be able to charge 30Kw of batteries on Octopus Go (4 hours over night). I have the money for an EV and like the look and price of the new Renault 5 EV which has V2L the larger of the two battery options is 52Kw. Will wait to see if Octopus will have a tariff that integrates with V2L and if Renault as a brand becomes accepted for Intelligent Go. Also having three of our largest windows changed from 20+ year old double glazed to triple glazed on 29th April 2024.

You have to sweat a battery system to get value out of it, that means cycling it fairly deeply every day, trying to cover winter loads is not economical and current prices.

ITS Technologies 9.5 kwh gen2 £2,634 without the VAT

Also you are saving, by potentially reducing the effect of inflation.

for that there are calculators you know what you are using and the cost from it.

I just bought a 5 kw lifepo4 battery for 870,- euro's

how about the cost of installation £300-£400

You also have to calculate that if you save £10,000 compounded at 5% interest for 10 years you would have nearly £16,500. So my view is definitely not worth it. You also have to add on how much it would cost to fit the extra battery.

cant you just sell the 5 and get another 9, may be cheaper and neater

Interesting ideas. However you have not included the opportunity cost of the batteries. If you had invested the cost of the batteries at, say 5% pa, the your numbers look very unattractive. If you do the sums on compound interest even worse. If it was an ISA then the tax free benefit erodes the benefit even more.

It didn’t look like you took account of the 10-20% energy conversion losses unfortunately incurred in charging and discharging the battery. I did my own similar calculations and came up with 5-22 years depending on which assumptions I made. Clearly not viable! Does make me wonder about original battery purchase, but there we are. Batteries are not good value. Extra solar panels are

Quick note: When comparing the battery prices, you assumed 4.2 working capacity on the 5.2 battery; but the full 9.5 on the 9.5 battery.