Guide
What size battery does a UK home need? 5kWh vs 10kWh
Updated
Bigger is not better, bigger is just dearer. A battery earns on the kWh it cycles, so the right size is the one your evening actually uses. Start from your bill, not the brochure.
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Start with your real usage
A typical UK household uses around 2,700 kWh of electricity a year, Ofgem's medium typical domestic consumption value, which averages out near 7.4 kWh a day (Ofgem, average energy use). Much of that lands in the evening, exactly when solar has stopped producing and peak rates apply, which is the slice a battery exists to cover. Your annual kWh is on your bill; divide by 365 and you have the only number that matters more than the price.
5kWh or 10kWh?
| Household | Sensible size | Typical installed cost |
|---|---|---|
| Low use, small household (under ~2,000 kWh/yr) | 3–5kWh | £2,500–£4,500 at the 5kWh point |
| Average home (~2,700 kWh/yr), no EV | 5kWh | £2,500–£4,500 |
| High use, EV or electric heating (4,000+ kWh/yr) | 10kWh+ | £4,500–£7,000 at the 10kWh point |
The per-kWh price falls as systems get bigger, which is why salespeople love the upsell, but an oversized battery cycles half-empty and the extra capacity never pays for itself. The honest test: would the second 5kWh actually be charged and used most days? For an average home without an EV, usually not. For an EV household on a cheap overnight tariff, usually yes, and the arbitrage case gets stronger too, see the tariff maths.
The details that change the answer
- Usable capacity vs nameplate: batteries hold back a reserve, so a 10kWh nameplate may deliver around 9–9.5 usable kWh depending on the model. Compare usable figures across quotes, it's the honest number.
- Solar size matters: the battery can only store what your panels spare after daytime use; a small array can't fill a big battery in winter, so match battery to array as well as to usage.
- Power rating (kW) is separate from capacity (kWh): the kW figure sets how much the battery can deliver at once; a low-power battery may not run an electric shower or oven alone, ask the installer what your peak loads need.
- Warranties are cycle-based as well as year-based, commonly around 10 years or a stated energy throughput, check the specific product's terms; a right-sized battery that cycles fully each day earns its keep well inside that window.
- Modular systems grow: several brands stack extra modules later, so starting at 5kWh and adding capacity if an EV arrives is a legitimate strategy, ask installers to price expandability.
A good installer sizes from your annual kWh, your tariff and your solar array, and shows the working. If a quote arrives without asking your usage, that's a brochure, not a design, get quotes from installers who ask.