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Batteries · Advice

What size home battery do I need?

Short answer

Start from how much electricity you use outside your cheapest hours, not a single rule. EST says a typical home battery is about 10 kWh, and Which? says average homes use 8-10 kWh a day. Many installers size slightly below daily peak-hour use so the battery cycles fully every day, then add modules later if needed.

Last checked 24 September 2026 · 9 sources

What should I know before deciding?

Installer rule (brief): daily use / 24 x 18 peak hours - assumes flat use; real homes use more in the evening. Also check inverter kW (can it supply your evening peak?) and charge kW (can it refill in the cheap window?). With solar, winter surplus is small, so size for the overnight-tariff role.

Is 'daily use ÷ 24 × peak-rate hours' a good way to size a home battery?

It's a quick first guess for time-of-use tariffs, but it assumes you use the same power every hour. Real UK homes use more in the evening, so about 86–91% of use falls outside a 5–6 hour cheap window, not 75%. For solar-only self-consumption, the MCS MGD 003 tables point to much smaller batteries, often 5–10 kWh.

Heuristic: capacity = daily kWh ÷ 24 × hours outside the cheap window (e.g. 24 ÷ 24 × 18 = 18 kWh). Profile method: capacity = daily kWh × PC1 share outside the window ÷ (1 − reserve). For 24 kWh/day and IOG: 24 × 0.879 = 21.1 kWh delivered, ~23 kWh usable with a 10% reserve. Power check: charge energy = delivered ÷ RTE (21.1 ÷ 0.9 = 23.4 kWh) must fit in the window at the charge rate (23.4 kWh ÷ 6 h = 3.9 kW). A 5 kW-charge Powerwall 3 fits; a 3 kW-charge battery would not fill in a 5-hour Go window (15 kWh max).

What does a typical UK home's electricity use look like across the day, for battery sizing?

The Elexon standard profile for ordinary homes (Profile Class 1) is low overnight and rises through the day. It peaks in the early evening, and a winter weekday peaks at about 17:30. About 12–15% of a day's use falls in a 6-hour overnight window and about 17–21% between 4pm and 7pm. Electric-heating homes (Profile Class 2) are very different: 40–50% of their use falls overnight.

Profile Class 1 = domestic unrestricted (single-rate) meters; Profile Class 2 = domestic Economy 7/off-peak meters (storage heaters, immersion). Source data are Elexon average-customer half-hourly shapes (1997 sample, published via UKERC/CEDA). Use them for the SHAPE, and scale them to the household's annual kWh. PC1 winter weekday: 12.3 kWh/day for the average customer, with a 0.92 kW half-hour peak at 17:30. Summer weekday: 8.9 kWh/day.

Can I add more battery capacity later?

Often, with modular systems, but check the rules first. Each enclosure is limited to 20 kWh and totals to 40 kWh (80 kWh outdoors or in detached buildings) under PAS 63100. Brands limit modules per stack or inverter and may require the same generation. More power may need a new DNO application.

Powerwall 3 datasheet: up to 3 Expansions per system (7 units total), up to 94.5 kWh and 44 kW with multiple PW3s.

Can my battery fill up in the cheap overnight window?

Only if its charging power is high enough. Divide usable capacity by charge power: 13.5 kWh at 5 kW takes about 2.7 hours, but 20 kWh at 3.6 kW needs over 5.5 hours. Cheap windows are often 4-7 hours, so check charge power, cold-weather limits and your main fuse.

Octopus Go cheap rate 00:30-05:30 (5 h); Intelligent Octopus Go guarantees 6 h 23:30-05:30. Grid-charging adds import load; keep within cut-out rating (G100 import limitation).

What to do next

Use your own half-hourly electricity data where possible. Compare usable kWh, kW, tariff rules and warranty terms before buying.

Use the battery calculator to test the numbers against your household.

Sources

Last checked: 24 September 2026