Two ways a home battery pays back: paired with solar (storing daytime generation for evening use) or standalone with a time-of-use tariff (charging cheap, using expensive). This calculator handles both.
Whole-house thinking, not just batteries
Batteries make sense when paired with the right tariff, the right solar setup and the right insulation. Get an assessment that looks at the whole system, not just one part.
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Battery savings come from two mechanisms:
1. Solar self-consumption boost: Without a battery, a typical UK household uses ~35% of generated solar directly. The rest is exported at ~15p/kWh (SEG). A battery captures the excess and uses it later, lifting self-consumption to ~80%. You save the difference between import rate (28p) and export rate (15p) on the shifted energy.
2. Tariff arbitrage: Time-of-use tariffs (Octopus Agile, Go, Flux; Economy 7) offer cheap overnight or off-peak rates (often 7–15p/kWh) and high peak rates (~30p+). A battery charged cheap and discharged peak saves the difference per kWh cycled.
Capacity assumptions: Usable capacity is 90% of nameplate (LFP chemistry). Round-trip efficiency 90%. Daily cycles: 1.0 for solar boost; 0.7 for standalone arbitrage (not always full cycle available).
Cost assumptions: Installed cost: 3 kWh £3,500; 5 kWh £4,500; 10 kWh £7,500; 13.5 kWh £9,500 (typical UK 2026 prices). Lifetime: 15 years, ~6,000 cycles.
Real savings depend heavily on tariff choice and usage patterns. The bigger lever for most households is the tariff, not the battery size.