The right battery covers your evening-to-morning consumption (plus a margin), not the biggest number the salesperson quotes. Oversized capacity cycles shallow, earns nothing on the marginal kWh and stretches payback badly.
Work from your usage: overnight+evening consumption (typically 40–60% of daily total) is what a battery actually serves. A home using 10kWh/day wants ~5–7kWh usable; 14kWh/day suggests 8–10kWh; heavy users and EV homes justify 12–15kWh+. With solar, also check summer surplus: capacity beyond what panels can refill just sits idle half the year.
| Household | Daily use | Suggested usable capacity |
|---|---|---|
| 1–2 bed, low use | 6–8 kWh | 4–6 kWh |
| 3 bed typical | 9–12 kWh | 6–9 kWh |
| 4 bed / working from home | 12–16 kWh | 9–12 kWh |
| 5 bed / EV / heat pump | 18–30 kWh | 12–20 kWh |
Usable capacity matters more than nameplate — check depth-of-discharge in the spec.
Each marginal kWh of capacity costs £350–£550 installed. If your evenings only drain 8kWh, the 14kWh unit’s last 6kWh rarely cycle — that’s £2,000–£3,000 of hardware earning pennies. The exceptions: agile-tariff arbitrage strategies that cycle aggressively, planned EV/heat-pump additions, and export-revenue plays where spare capacity sells into peak prices.
Most quality systems are modular — stackable units let you start at 6–10kWh and add capacity later if real usage data justifies it. Buying the expansion option costs little; buying unused kilowatt-hours upfront costs a lot. Get winter and summer scenarios modelled in any quote, and see our battery storage guide for cost benchmarks.
Facts checked 21 August 2026. We link to official sources and show when facts were last verified. If a scheme changes, we update the page.
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