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Solar + battery savings calculator
See how much a home battery could add to your solar savings by storing daytime power for the evening, and whether it pays back on its own.
Defaults from our data file, last updated 2026-10-20.
Estimates only, not financial advice. Results use average UK figures and the assumptions shown. Your real costs depend on your home, tariff, region and installer quotes. Always get several quotes from certified installers and check current tariff rates before deciding.
How a battery changes the maths
Without a battery, any solar you don't use straight away is exported, usually for far less than you pay to import. A battery captures some of that surplus and releases it in the evening, so each stored unit is worth the difference between your import price and your export rate. With an import price of 26.32p and an export rate of 12p, each kWh moved through the battery is worth about 14p. With a 4.1p export tariff it's worth about 22p, so a battery makes more sense when export rates are low.
The calculator assumes a battery can store and release about 90% of its usable capacity (allowing for round-trip losses) on around 250 days a year. On dull winter days there often isn't enough surplus solar to fill it. The amount it can shift is capped at the solar you would otherwise have exported. It then shows three results: the extra saving from the battery, the payback of the whole system, and the payback of the battery on its own.
Defaults and assumptions
- Battery cost: about £450 per kWh plus £1,200 for installation and hybrid inverter work, so £3,450 for 5 kWh. This is an editorial estimate. Batteries fitted with or added to solar are zero-rated for VAT in Great Britain until 31 March 2027.
- Electricity price: 26.32p per kWh, the Ofgem price cap average for October–December 2026.
- Export rate: 12p per kWh by default. Change it to match your tariff.
Worked example
A 4 kWp system in south-west England generates about 4,000 kWh. At 45% self-use, around 2,200 kWh would be exported. A 5 kWh battery can shift up to about 1,125 kWh a year (5 × 0.9 × 250). At a 14.3p gap between import and export that adds about £161 a year, so the battery alone takes over 20 years to pay back, which is longer than most battery warranties of around 10 years. With a 4.1p export rate the gap becomes 22.2p and the battery adds about £250 a year, so payback falls to around 14 years. On solar savings alone, a battery is often marginal.
When batteries make more sense
- Time-of-use tariffs. Many battery owners charge from the grid overnight at cheap rates (for example under 10p on some EV tariffs) and use that power at peak times. This "arbitrage" can earn far more than solar shifting, especially in winter. This calculator doesn't model it. Use our smart tariff comparison to see what cheap off-peak rates are worth.
- Peak export tariffs. Some tariffs pay more for export in the early evening (for example 16p from 4pm to 7pm), which rewards batteries.
- Fitting it with the panels. Adding the battery at the same time saves a second visit and may let you use a cheaper hybrid inverter.
- High evening use, such as families who are out all day and use a lot of power after 5pm.
Things to check before buying
Check usable capacity rather than headline capacity, the warranty length and guaranteed capacity at the end of it (often 70% after 10 years), and whether the battery supports backup during power cuts. That often needs extra wiring. Ask installers to model your actual half-hourly usage from your smart meter data, which is much more accurate than any average-based calculator, including this one.
FAQs
Is a solar battery worth it in the UK?
On solar self-use alone, often only just, as payback can exceed the battery warranty. Batteries look better with low export rates or time-of-use tariffs that let you charge cheaply overnight.
What size battery do I need?
A common rule of thumb is roughly 1–1.5 kWh of usable storage per kWp of solar, or enough to cover your typical evening use. Bigger batteries sit part-empty for much of the winter.
Can I add a battery to existing solar panels?
Yes. An AC-coupled battery can be added to most existing systems. Check whether this affects any existing export or Feed-in Tariff arrangements first.
How long do home batteries last?
Most lithium iron phosphate batteries carry 10-year warranties and are rated for several thousand cycles, with capacity slowly declining over time.
Related calculators
Sources
- Ofgem – Energy price cap unit rates and standing charges (1 Oct–31 Dec 2026)
- Octopus Energy – Outgoing Octopus export tariffs
- Smart Export Guarantee rates compared, October 2026 (solarbatterygrants.co.uk)
- HMRC – VAT on energy-saving materials and heating equipment (Notice 708/6)
- European Commission JRC – PVGIS solar irradiance tool
- MCS – Find a certified installer
Estimates only, not financial advice. Results use average UK figures and the assumptions shown. Your real costs depend on your home, tariff, region and installer quotes. Always get several quotes from certified installers and check current tariff rates before deciding.