Advanced options
The model: charge cheap overnight (or from solar excess), run the house on it through peak. The gap input is everything: standard-vs-EV-tariff gaps of 15-20p make batteries work: without a time-of-use tariff there's no arbitrage and no payback. Batteries carry 10-12 year warranties (~6,000-10,000 cycles): a payback longer than the warranty is the polite way of saying "not yet". Battery-only installs attract 0% VAT (as do solar+battery).
What The Home Battery Calculator Works Out
- Saving per year: kilowatt-hours shifted each day multiplied by the gap between your day and night rates, times 365. Eight kWh across a 13.5p spread banks around GBP 390 a year; the gap is everything, because without a time-of-use tariff there is no arbitrage.
- Payback: the installed cost divided by the yearly saving. Popular 10 kWh to 13 kWh units cost GBP 4,500 to GBP 7,000 installed, giving 12 to 15 year paybacks at standard gaps, which sits uncomfortably against 10 to 12 year warranties. Battery-only and solar-plus-battery installs are zero-rated for VAT.
- The honest position: batteries also buy blackout cover (only if the changeover hardware is installed), solar self-consumption and grid-flex income, all of which the simple arbitrage line understates, while falling hardware prices mean waiting has rarely been punished.
- Round-trip efficiency (advanced): the share of stored energy you get back out, typically 85% to 95% for lithium systems. Defaults to 100% so the basic answer is unchanged; lowering it trims the saving in proportion.
- Extra income per year (advanced): export payments, flexibility scheme earnings or the value of solar you would otherwise have exported at a 15p SEG rate and imported back at the full unit rate. Added to the yearly saving and payback.
- Capacity loss per year (advanced): batteries fade, typically 1% to 3% a year. Applied year by year to the lifetime saving so late-warranty years earn less.
- Net gain over the warranty (advanced): total savings across the warranty years minus the purchase price. A negative figure means the battery has not repaid itself before the guarantee runs out.
- Cycle life (advanced): daily kWh shifted divided by usable capacity gives cycles per year; the warranty cycle limit divided by that gives the years until the cycle cap bites. Most warranties end at whichever of calendar years or cycles comes first.
- Cost per kWh stored (advanced): the hardware price spread over every kilowatt-hour it will shift in its warranted life. If this is higher than your tariff gap the battery loses money on every cycle.
- Gap needed to pay back in warranty (advanced): the day-night spread in pence that would repay the battery exactly within the warranty, so you can see how far your tariff is from making the maths work.