Don't get solar batteries

Shiftyxx

The maths is simple. If you DON'T have panels, you can sell the electricity you generate to EON for 16.5p/kWh. When you later need the power, you can buy it back for 21p inc VAT (as at Aug 2025, also from EON). So it'll cost you the difference, which is 4.5p/kWh. If you DO have batteries, you'll use your own power and save that 4.5p / kWh. So a 3kW battery will save you a maximum of 13.5p a day, (assuming it gets fully charged every day, which it won't in the Winter). In one year, this works out to a maximum of £49. In reality, it'll be less than that as some Winter days generate less than 2kWh in the whole day - So let's assume batteries save you £40/year. If the battery costs £5000, they you should get your money back in around 125 years - assuming the battery lasts 125 years, which of course, it doesn't!. So if you buy the battery when you are 20, and you live to 150, and the battery is still working, you'll be in profit. Good luck with that then !!!

Shiftyxx

Sorry - serious typo in that. DO get solar panels. DON'T get solar batteries.
The panels save you loads. The batteries cost you loads.

Shiftyxx

What I MEANT TO SAY (TO CLARIFY) IS 
The maths is simple. If you DON'T have BATTERIES, you can sell the electricity you generate to EON for 16.5p/kWh. When you later need the power, you can buy it back for 21p inc VAT (as at Aug 2025, also from EON). So it'll cost you the difference, which is 4.5p/kWh. If you DO have BATTERIES, you'll use your own power and save that 4.5p / kWh. So a 3kW battery will save you a maximum of 13.5p a day, (assuming it gets fully charged every day, which it won't in the Winter). In one year, this works out to a maximum of £49. In reality, it'll be less than that as some Winter days generate less than 2kWh in the whole day - So let's assume batteries save you £40/year. If the battery costs £5000, they you should get your money back in around 125 years - assuming the battery lasts 125 years, which of course, it doesn't!. So if you buy the battery when you are 20, and you live to 150, and the battery is still working, you'll be in profit. Good luck with that then !!!

badmemory

I think you have seriously misunderstood.  I use around 20kWh per day some of which is gas.   I always use at least 2kWh per day from the grid as I have one large daily usage that even the solar plus the battery cannot fill.  Also any gas I may use, heating & cooking etc.  The last 3 months my supplier has been paying me.  So even if I get nothing else the other months of the year then I will be saving at least £1k a year, which is just below what I would need for the 10 year payback time they say.  But a lot of the other months will involve some saving.  Also on real winter days it is possible to fill your battery on cheap rate overnight to use during the day, but I haven't got there yet.

EricMears

badmemory said:

I think you have seriously misunderstood.  I use around 20kWh per day some of which is gas.   I always use at least 2kWh per day from the grid as I have one large daily usage that even the solar plus the battery cannot fill.  Also any gas I may use, heating & cooking etc.  The last 3 months my supplier has been paying me.  So even if I get nothing else the other months of the year then I will be saving at least £1k a year, which is just below what I would need for the 10 year payback time they say.  But a lot of the other months will involve some saving.  Also on real winter days it is possible to fill your battery on cheap rate overnight to use during the day, but I haven't got there yet.

I agree with 'badmemory' !

I've had batteries for about a year now and the electricity I've had to import since start of this year has cost me an average of 7.8p/kWh.
(I don't sell any electricity as I'm on deemed exports and just make sure I use as much as possible)

Martyn1981

Shiftyxx said:

What I MEANT TO SAY (TO CLARIFY) IS 
The maths is simple. If you DON'T have BATTERIES, you can sell the electricity you generate to EON for 16.5p/kWh. When you later need the power, you can buy it back for 21p inc VAT (as at Aug 2025, also from EON). So it'll cost you the difference, which is 4.5p/kWh. If you DO have BATTERIES, you'll use your own power and save that 4.5p / kWh. So a 3kW battery will save you a maximum of 13.5p a day, (assuming it gets fully charged every day, which it won't in the Winter). In one year, this works out to a maximum of £49. In reality, it'll be less than that as some Winter days generate less than 2kWh in the whole day - So let's assume batteries save you £40/year. If the battery costs £5000, they you should get your money back in around 125 years - assuming the battery lasts 125 years, which of course, it doesn't!. So if you buy the battery when you are 20, and you live to 150, and the battery is still working, you'll be in profit. Good luck with that then !!!

What about charging the battery on cheap rate in the winter to power the house, and avoid buying at day rate?

Last winter our most expensive month was January with a bill of £110*. That was for normal leccy demand, space heating (HP powered by batts (and a bit of PV)), DHW, cooking and transport (BEV's). Leccy import was ~1,100kWh, almost all cheap rate at 7p/kWh, about 14kWh was at day rate (~25p/kWh).

Also, and I appreciate this is an edge case, we couldn't get permission from the DNO for more export. But by having batts on the DC side (as a buffer), we were able to upgrade 3.58kWp of solar behind a 3.7kW inverter, with 6.75kWp of PV behind a 3.68kW limited hybrid inverter. I know that's not what you were saying, but the battery allows us to generate far more per year than we otherwise could. That extra generation displaces more import, and earns more export.

*That includes gas cost, approx 100kWh of gas which should move to leccy consumption at a roughly 1:1 unit cost, but save ~£10pm in standing charges. So £100 for Jan is a reasonable assumption estimating forward. Currently total leccy and gas for the year should be £650 to £700, with export income of ~£600.

NedS

My battery offers full home backup so automatically kicks in and allows me to continue to run the house throughout a power cut - priceless!

Croft12

Shiftyxx said:

The maths is simple. If you DON'T have panels, you can sell the electricity you generate to EON for 16.5p/kWh. When you later need the power, you can buy it back for 21p inc VAT (as at Aug 2025, also from EON). So it'll cost you the difference, which is 4.5p/kWh. If you DO have batteries, you'll use your own power and save that 4.5p / kWh. So a 3kW battery will save you a maximum of 13.5p a day, (assuming it gets fully charged every day, which it won't in the Winter). In one year, this works out to a maximum of £49. In reality, it'll be less than that as some Winter days generate less than 2kWh in the whole day - So let's assume batteries save you £40/year. If the battery costs £5000, they you should get your money back in around 125 years - assuming the battery lasts 125 years, which of course, it doesn't!. So if you buy the battery when you are 20, and you live to 150, and the battery is still working, you'll be in profit. Good luck with that then !!!

Its not so simple. eg if you are on IOF you might export >30kwh/d at this time of year at 26p average -v- 15p

ed110220

Shiftyxx said:

The maths is simple. If you DON'T have panels, you can sell the electricity you generate to EON for 16.5p/kWh. When you later need the power, you can buy it back for 21p inc VAT (as at Aug 2025, also from EON). So it'll cost you the difference, which is 4.5p/kWh. If you DO have batteries, you'll use your own power and save that 4.5p / kWh. So a 3kW battery will save you a maximum of 13.5p a day, (assuming it gets fully charged every day, which it won't in the Winter). In one year, this works out to a maximum of £49. In reality, it'll be less than that as some Winter days generate less than 2kWh in the whole day - So let's assume batteries save you £40/year. If the battery costs £5000, they you should get your money back in around 125 years - assuming the battery lasts 125 years, which of course, it doesn't!. So if you buy the battery when you are 20, and you live to 150, and the battery is still working, you'll be in profit. Good luck with that then !!!

You're forgetting that a) time of use/export tariffs exist and b) you can store cheap rate electricity from the grid with a battery not just solar. This is especially important during the darker part of the year when solar is yielding little and even more so if you use a lot of electricity in winter, say for heating.

I last did my calcs in late May for the year so far. I'm on Octopus Go so imports are 8.5p 00:30 - 05:30 and I can avoid the 29.17p rate the rest of the time using the battery. Exports are 15p outside that window.

So far this year the Octopus app says I've imported £325.47 worth of electricity and exported £306.28 so a balance of £19.19. My Growatt app says our load consumption has been 1795 kWh. Octopus's flat rate tariff is 25.38 p/kWh with a standing charge of 52.90 p/day. Therefore I would have spent £455.57 + £77.23, so have saved £513.61 so far this year [over no solar or battery].

Now if I had the solar but no battery and assume Octopus's flat rate tariff and consuming 30% of my generation and exporting 70% then my 1830 kWh of solar would have exported £192 and imported £325 with a standing charge of £77 so I would have paid £210. Given I actually paid £19, I'm £191 better off over 5 months with the battery. And it's worth noting that I didn't have the battery set up optimally for the start of the year, so I'm expecting the savings to be greater over the full year.

In winter I could easily use 17 kWh in 24h. If we ignore the solar because it will be low, that's <£1.70 of imports. With just the solar, again ignoring the low solar yield, the flat rate would have cost ~£4.30, so a saving of >£2.60/day.

QrizB

ed110220 said:

You're forgetting that a) time of use/export tariffs exist and b) you can store cheap rate electricity from the grid with a battery not just solar. This is especially important during the darker part of the year when solar is yielding little and even more so if you use a lot of electricity in winter, say for heating.

This, so much this.

I've just checked my bill for February 2025. We have gas heating but, between house and EV, we used about 500kWh of electricity that month. Thanks to IOG and our Fogstar battery the average price of our electricity was about 8p/kWh, so £40 + SC for the month.

badmemory

When I have a full year under my belt knowing what I am doing, which always helps,  I expect to have easily saved 10% of my installation costs as promised.  As I am fully expecting energy costs to keep going up I expect to easily save the costs of installation & that is bearing in mind that my site is not exactly optimal.  East/west & in winter a lot in permanent shadow.