Last reviewed: 5 September 2026. Default peak price is the Ofgem price cap for 1 October to 31 December 2026 (26.32p/kWh, direct debit, GB average). The VAT treatment follows HMRC Notice 708/6. Nothing is sent anywhere: everything runs in your browser.
| Year | Delivered (kWh) | Saved | Cumulative |
|---|
How the payback is worked out
A battery does not generate electricity. It moves it in time: it buys when power is cheap and releases it when power is dear. So the whole return comes from one number, the gap between the two prices, minus the energy lost on the round trip.
- Each kilowatt-hour delivered has cost you 1 ÷ 0.9 = 1.11 kWh bought at the off-peak rate, because we assume a 90% round-trip efficiency.
- The value of each delivered kilowatt-hour is therefore the peak price minus that charging cost, not the peak price itself. Ignoring losses inflates the answer by roughly a tenth.
- Delivered energy each year is usable capacity × cycles, reduced by capacity loss as the cells age.
- The bill is the quoted price plus VAT at whichever rate applies on the day you buy.
The assumptions, stated openly
| Assumption | Value | Where it comes from |
|---|---|---|
| Peak electricity price | 26.32p/kWh | Ofgem price cap, 1 Oct–31 Dec 2026, direct debit, GB average |
| VAT until 31 Mar 2027 | 0% | HMRC Notice 708/6, energy-saving materials |
| VAT from 1 Apr 2027 | 5% reduced rate | HMRC Notice 708/6. Not 20% |
| Round-trip efficiency | 90% | Our own working assumption, AC-coupled system |
| Cycles per year | Your input, 340 default | Our own assumption: not every night is a full cycle |
| Electricity price rise | 0% unless you change it | Deliberate. We do not forecast the market |
[VERIFICAR: the 90% round-trip efficiency and the default cycle count are our own reasoned assumptions rather than figures published for the UK installed fleet. We have not found a primary source that publishes an average, and we will not present an estimate as one.]
No export income is counted anywhere in this calculation. A self-installed plug-in kit cannot access the Smart Export Guarantee, so any surplus is given away for nothing. Calculators that add export revenue to a plug-in setup are describing a system you do not have.
What a sensible answer looks like
If the payback lands comfortably inside the warranty period, the case is reasonable. If it lands just outside, the battery is a lifestyle purchase rather than an investment, and it is honest to call it that. If it never pays back at all, the calculator will say so rather than stretching the assumptions until it does.
Before committing, size the unit properly with the battery size calculator, because buying more capacity than you cycle is the most expensive mistake in this market, and read what the VAT change actually costs you if you are thinking of waiting.