Battery Sizing Calculator
Work out how much capacity your usage actually justifies and what off-peak charging would save, with or without solar. Defaults are Ofgem's typical consumption values and the Ofgem price cap, 1 October to 31 December 2026, checked 20 September 2026.
Ofgem multi-rate typical values: low 5.2, medium 9.3, high 16.7 kWh a day.
Shiftable load
5.4 kWh/day
Suggested usable capacity
6.5 kWh
Est. annual saving
£342
Hardware-only payback
14.6 yrs
Installation is extra. Compare with the warranty length.
Off-peak default is Intelligent Octopus Go (23:30–05:30), which needs a compatible EV or charger. Enter a postcode for your region's rates.
Assumptions
- Consumption
- Ofgem 2026 multi-rate typical values; 58% of use outside the cheap window (Economy 7 split) (Ofgem TDCV decision)
- Day rate
- 26.32p/kWh, Ofgem price cap, 1 October to 31 December 2026; enter a postcode above the calculator for your region's Octopus rates (Ofgem)
- Off-peak rate
- 8p/kWh, Intelligent Octopus Go 23:30–05:30; enter a postcode above the calculator for your region's Octopus rates (Octopus)
- Round trip
- 89% best case (Tesla Powerwall 3 datasheet, DC-coupled path) (Tesla)
- Not modelled
- Installation cost, battery degradation, backup value
Rates checked 20 September 2026. Check them again before you commit to anything.