What Size Home Battery Do I Need? kWh Sizing Without the Sales Pitch
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Start with one number: your evening usage
A battery earns its keep by covering the hours you'd otherwise buy expensive grid electricity — typically 4pm to midnight. Look at your smart meter data (or your annual kWh ÷ 365): a typical UK home uses 8–12kWh a day, with roughly half in that evening window.
The honest sizing bands
- 5kWh (e.g. GivEnergy 5kWh) — covers the evening peak for a lower-usage home or a smaller solar array's excess. The right entry point for flats and 1–2 person households.
- 9–10kWh (e.g. GivEnergy 9.5kWh) — the sweet spot for a typical family: stores enough cheap overnight electricity to run most of the day, or banks a 4kW solar array's summer excess.
- 13.5kWh (Tesla Powerwall 3) — high-usage homes, heat pump owners, EV households, or anyone wanting whole-home backup through power cuts (the Powerwall's party trick).
The two ways a battery makes money
1. Tariff arbitrage (no solar needed). Charge at a cheap overnight rate (~7p/kWh), use it during the day instead of 25p+ grid power. A 9.5kWh battery cycling daily shifts ~£2.50–3.50 a day — £700–£1,200 a year depending on usage and tariff. This is why batteries now make sense without solar — and installed battery storage is 0% VAT either way.
2. Solar self-consumption. Without a battery, a working household exports half their solar generation for pennies. A battery keeps it for the evening. Pair sizing to your array: roughly 1–1.5kWh of battery per kW of solar is a sensible starting ratio.
When bigger stops paying
A battery you never fully cycle is capital sitting on a wall. If your evening usage is 6kWh, a 13.5kWh unit spends most of its life half-used — the payback maths favours the smaller unit. The exceptions: backup power requirements, an incoming heat pump or EV (size for next year's usage, not last year's), or time-of-export tariffs that reward dumping a full battery back to the grid at peak — ask us about those at survey.
Chemistry and lifespan, briefly
Modern home batteries (LiFePO4) comfortably run 10+ year warranties and thousands of cycles — daily cycling is what they're designed for, so don't baby them.