Battery size optimiser
5 kWh vs 10 kWh battery with 7 kWh/day available to shift
A larger battery only adds value when enough energy is available to fill and discharge the extra capacity. This example compares £4,000 for 5 kWh with £6,000 for 10 kWh.
Illustrative 18p/kWh value spread · 90% efficiency16.9 years
Extra battery cost £2,000 · extra annual value £118
Tariff/value-spread sensitivity
| Value spread | 5 kWh annual value | 10 kWh annual value | Extra annual value | £2,000 extra-cost payback |
|---|---|---|---|---|
| 10p/kWh spread | £164/yr | £230/yr | £66/yr | 30.4 yrs |
| 18p/kWh spread | £296/yr | £414/yr | £118/yr | 16.9 yrs |
| 25p/kWh spread | £411/yr | £575/yr | £164/yr | 12.2 yrs |
“Available to shift” is the usable daily energy opportunity, not total household consumption. Real solar surplus, cheap-rate availability, battery power limits and degradation determine utilisation.