Home energy guide
What size home battery do I need?
Size your battery to the electricity you use at pricey times, not your whole day. For lots of homes that's 5 to 10 kWh. Bigger isn't better if half of it just sits there empty.
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Work out your own numbers with the home battery calculator →
Start with your "expensive hours"
A battery only saves money on the power it supplies when you'd otherwise pay full price. So the right size is roughly what you use outside your cheap tariff window, or after the sun goes down.
A typical medium-use home uses 2,700 kWh a year (Ofgem, not re-checked today). On a time-of-use tariff, that works out at roughly 6.6 kWh of usable battery to cover the day and evening.
See the numbers
- 2,700 kWh a year ≈ 7.4 kWh a day.
- If 20% is already used overnight on a time-of-use tariff, about 5.9 kWh a day is used in the day and evening.
- At 90% battery efficiency, delivering that needs about 6.6 kWh of usable capacity.
Find your own numbers
- Smart meter data. Many supplier apps show half-hourly use. Add up a typical winter day outside your cheap window.
- Seasons. You use more in winter, especially with electric heating or a heat pump. In summer you use less but have more spare solar.
- Big loads. Note the power-hungry stuff: oven, kettle, shower, heat pump. A battery's kW rating limits how much it can supply at once.
Sizing with solar
With solar, the battery fills up from spare sunshine in the day and empties in the evening. Make it bigger than your typical summer surplus or evening use and it'll never fill or empty fully.
Seasons make this tricky: a battery that's full every day in June may barely charge in December. A night-rate tariff can top it up in winter.
Storing a unit of solar is worth about 13p; shifting a unit of cheap night power is worth about 19.1p (see our savings guide). So many owners do both: solar in summer, cheap overnight charging in winter.
See the numbers
- A 4.5 kWp system in London makes about 4,586 kWh a year (PVGIS), far more in summer than winter.
- Stored solar: 26.32p saved, minus 12p export lost and some losses ≈ 13p per kWh.
Examples
Roughly: a small flat suits about 3 kWh, a medium home about 7 kWh, and a large home or one with a heat pump about 11 kWh.
See the numbers
| Household | Daytime and evening use | Usable capacity that covers it | Yearly saving from night shifting (300 cycles) |
|---|---|---|---|
| Small flat | 3 kWh | about 3.3 kWh | £172 |
| Medium home | 6 kWh | about 6.7 kWh | £344 |
| Large home or heat pump | 10 kWh | about 11 kWh | £573 |
Saving = kWh delivered × (26.32p − 6.5p ÷ 0.9). 300 full cycles a year is an assumption; many batteries do fewer. These figures assume the day rate equals the cap average; on many smart tariffs it's higher, which raises the saving, while a lower day rate reduces it.
The cost of going too big
Extra capacity costs money but only earns when it's used. Put a 13 kWh battery in a medium home with 6 kWh of daytime use and the extra half mostly sits idle. That changes if you add an EV, a heat pump or an export tariff that pays you to discharge at peak times.
Before going bigger, divide the extra cost by the extra yearly saving.
When bigger makes sense
- a heat pump or electric heating with lots of winter evening use;
- an EV charged in the day when it's at home but the solar isn't enough;
- a tariff that pays well for exporting at peak times, such as Octopus Flux (see battery tariffs);
- backup power for a home with frequent power cuts.
Start small, grow later
Some batteries let you add modules later. Start small, collect a year of data, then add more if it's worth it. Ask whether the inverter and warranty allow it, and what a module costs.
Don't forget power
A 10 kWh battery that only discharges at 3 kW can't run a 9 kW electric shower; the rest comes from the grid at full price. Check the continuous kW rating against your evening loads.
Try different sizes in the home battery calculator.
Frequently asked questions
What size battery do I need for a 3-bed house?
It depends on use, not bedrooms. Add up the electricity you use outside your cheap tariff window or after dark; many homes fall between 5 and 10 kWh usable.
What size battery for 4 kWp of solar?
Roughly your evening use or your typical summer daily surplus, whichever is smaller. For many homes that is 5–10 kWh.
Is a bigger battery better?
Only if you use the extra capacity. Unused capacity adds cost without saving money.
Can I add more battery capacity later?
With some modular systems, yes. Check the inverter and warranty first.
Related guides
- Home battery savings: night rates and stored solar
- How much does a home battery cost in the UK?
- Is a home battery worth it without solar panels?
- Tariffs for home batteries: overnight, three-rate and export tariffs
- Home batteries in Guernsey and Jersey: do the numbers work?
- All home-energy calculators
Sources (checked 27 October 2026 unless stated)
- Ofgem typical domestic consumption values (2,700 kWh): not re-checked today
- EU JRC PVGIS 5.3 (queried 27 Oct 2026 for our solar guide): London 1,019 kWh per kWp
- Ofgem: price cap 1 Oct–31 Dec 2026 (published 26 Aug 2026): 26.32p/kWh
- Octopus Energy: Intelligent Octopus Go: from 6.5p/kWh (checked 31 Oct 2026)
- Octopus Energy: Outgoing Octopus: 12p export
- 90% round-trip efficiency and 300 cycles a year: our assumptions