Home Battery Calculator
What size battery your evening usage needs, and whether the arbitrage justifies the cost.
A home battery is worth the difference between what you paid to fill it and what you avoid paying to empty it.
How the home battery calculator works
A home battery is worth the difference between what you paid to fill it and what you avoid paying to empty it. Charged from surplus solar you would otherwise export, each stored unit is worth the retail rate minus the feed-in rate — a much larger spread than charging from the grid off-peak.
Usable capacity is less than nameplate. Depth of discharge is typically 90% for lithium, and round-trip efficiency around 90%, so a 13.5 kWh battery delivers roughly 10.9 kWh to the house.
The honest arithmetic is often unflattering: at current prices, batteries frequently pay back beyond their warranty period on economics alone. People buy them for backup power and independence too, and this page cannot price those.
Formula: usable = capacity × depth of discharge × round-trip efficiency
Worked examples
| Inputs | Usable capacity | Note |
|---|---|---|
| A 13.5 kWh battery | 10.94 kWh of 13.5 nameplate | 10.9 kWh usable — short of 12 |
| Sized to cover the evening | 12.15 kWh of 15 nameplate | 12.15 kWh usable |
| A wider tariff spread | 10.94 kWh of 13.5 nameplate | much faster payback |
FAQFrequently asked questions
How big a battery do I need?
Enough usable capacity to cover evening and overnight use. Divide that by depth of discharge and efficiency to get the nameplate size.
Why is usable capacity less than nameplate?
Depth of discharge protects cell life, and round-trip efficiency loses energy in conversion. Together they take roughly 20%.
Do home batteries pay for themselves?
On arbitrage alone they often pay back beyond their warranty at current prices. Backup power and independence are real benefits this cannot price.
What makes payback faster?
A wide gap between the retail rate and the feed-in rate. That spread is what each stored unit is worth.
Does one cycle a day happen in practice?
Rarely in winter, when there is little surplus to store. The annual figure here is an upper bound.
Where these figures come from
- International Energy Agency — Energy Efficiency — appliance consumption and standby power context
- Ofgem — the UK energy regulator
Last checked: September 2026. Appliance power figures are typical ratings; check the label on your own equipment, which is authoritative for it.