How it works
A battery earns its keep by moving electricity from when it is cheap to when it is expensive. With solar, it stores power you would otherwise export for a low credit and gives it back in the evening, when you would pay the full price. On a time-of-use or EV tariff, it can also charge from cheap overnight power and run the house at peak times.
Each kWh stored is worth the price you avoid (less about 10% lost in charging and discharging) minus what that kWh would have earned or cost. The calculator adds that up over a year, grows it with electricity prices, shrinks it as the battery slowly loses capacity, and compares the running total with the battery's price after rebates.
The key test is the warranty: most home batteries are guaranteed for 10 to 15 years. A battery that takes longer than that to pay back is a gamble on savings alone, though backup during outages may still make it worth it to you.
The formula
solar stored = min(spare solar a day, capacity) × sunny days
off-peak top-up = (capacity − solar stored each day) × days (only on a time-of-use tariff)
yearly saving = solar stored × (0.9 × price avoided − export credit)
+ top-up × (0.9 × price avoided − off-peak price) + grid-service payments
payback = first year the running total of savings passes the cost after rebates
Worked examples
All with 2.5% capacity loss and 3% electricity price rises a year.
| Situation | First-year saving | Payback |
|---|---|---|
| US: 13.5 kWh for $13,000, storing 8 kWh of solar on 300 days, flat 18.2¢ price, 5¢ export credit | $273 | over 20 years |
| Same battery, but a 40¢ evening peak price | $744 | 14.3 years |
| UK: 10 kWh for £7,000, no solar, charging at 8p overnight and avoiding 26.32p | £573 | 11.9 years |
The same battery can be a poor investment on a flat tariff and a reasonable one where peak and export prices are far apart. That gap matters more than the battery's brand.
Frequently asked questions
How long does a home battery take to pay for itself?
Typically 10 to 15 years where there is a big gap between what you pay at peak and what you get for exports, and never on savings alone where electricity prices are flat and export credits are close to retail.
Is a solar battery worth it?
It depends on your tariff more than the battery. See is a home battery worth it for when it pays and when it doesn't, and is solar still worth it in 2026 for the panels themselves.
Can a battery save money without solar?
Yes, on a tariff with cheap overnight electricity. In the UK example above, charging at 8p and avoiding 26.32p saves about £573 a year.
What is a virtual power plant payment?
Some utilities and retailers pay battery owners, either a yearly amount or per event, to draw power from their batteries at times of peak demand. Add it in the grid-service payments box.
How long do home batteries last?
Most carry a 10 to 15 year warranty guaranteeing about 70% of the original capacity at the end. Lithium iron phosphate batteries often last longer than their warranty.
Sources
- Battery prices and incentives: Powerwall 3 installed quotes run about $10,000 to $15,400 (2026); the 30% federal credit ended after 2025; 10 kWh home batteries typically £6,000 to £10,000 installed with 0% VAT (until 31 March 2027); Powerwall 3 quotes about $11,000 to $17,000 before the federal discount; the discount is about $252 per usable kWh (May–Dec 2026).
- Electricity prices: US average residential price, EIA 2026 forecast (checked Oct 2026); Ofgem price cap unit rate, 1 Oct – 31 Dec 2026, Direct Debit (VAT on electricity is 0% until 31 Mar 2027).
- Clean Energy Regulator, Cheaper Home Batteries Program.
Estimates are for planning only and are not financial advice. Get written quotes before you buy.