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Batteries
Batteries · Advice

How does a battery save money in winter?

Short answer

Fill it on the cheapest overnight rate and let it carry the house through the day and the 4-7pm peak. With a 6-hour window at 7-9p and a battery sized to most of your daily use, you can avoid most peak-rate imports. Make sure the charge window, clock and target percentage match your tariff.

Last checked 24 September 2026 · 11 sources

What should I know before deciding?

Checked overnight rates (24 Sep 2026, existing tariff rows): Octopus Go 8.63p 00:30-05:30; Intelligent Octopus Go ~8p 23:30-05:30; EDF GoElectric 6.99p 23:00-06:00; E.ON Next Drive 6.7p 00:00-07:00 (Mar 2026); British Gas ~7.9-9p 00:00-05:00. Most need an EV. Typical owner pattern reported by installers: full by 6am, 70-80% through the day, ~50% by evening, empty around midnight. Check charge power: a 3 kW charger fills only ~15-18 kWh in a 5-6 hour window. Keep the battery frost-free, or charging may be throttled.

How much solar will reach my battery in winter?

Very little. A south-facing 35° roof near London makes about 36 kWh per kWp in December against 123 in June, under 30%. Near Glasgow it's about 17% of July. A 4 kWp array averages only ~4.6 kWh a day in a London December, and the house mostly uses it as it's generated. Winter battery value therefore comes from cheap-rate charging.

PVGIS 5.3 long-term monthly (south, 35°): London Dec 36 / Jun 123 kWh/kWp; Glasgow Dec 19 / Jul 112 kWh/kWp. December is only 3-4% of annual output in southern England. Simulator: use monthly kWh/kWp (PVGIS) × a daily PV shape. Charge the battery only from surplus over the PC1 load, then add grid charging in the cheap window.

What size battery and savings would a home using 24 kWh a day get on a cheap overnight tariff?

On Intelligent Octopus Go (8p off-peak, ~30p peak), about 21 kWh of a 24 kWh day falls outside 23:30–05:30. A battery holding about 23 kWh usable, charging at 4 kW or more, could shift it. That saves roughly £4.50 a day (~£1,600 a year) and cuts the unit cost to about a third. An 18 kWh battery saves about £3.80 a day.

Inputs: 24 kWh/day, PC1 winter-weekday shape, IOG 8p (page) off-peak / 30.09p peak (API region C), 44.12p/day standing, RTE 0.90, 10% reserve. No battery on IOG: 21.1 × 30.09p + 2.9 × 8p = £6.58 + standing 44p = £7.02/day. 23 kWh usable battery: (21.1 ÷ 0.9) × 8p + 2.9 × 8p = £2.10 + 44p = £2.54/day → saving £4.48/day ≈ £1,635/yr.

Why does my battery charge slowly or not at all in cold weather?

Lithium cells must not be charged hard when cold, so the battery management system slows or blocks charging. Tesla's Powerwall 3 limits charging between 0 and 10 C and blocks it below 0 C unless heated; GivEnergy and SigEnergy block charging below about 0 C. Discharging still works, so the battery may empty but not refill overnight.

Charging a cold graphite anode risks lithium plating, a permanent capacity loss; BMSs therefore derate or stop charging near 0 C. Powerwall 3: normal above 10 C; charging limited 0-10 C; blocked below 0 C; Heat Mode warms cells with bonded heaters, predicting from 7 days of history; ~200 Wh at -20 C (secondary summary of Tesla white paper). AlphaESS: below 10 C cells 'begin to lose performance, and at 0 C charging often stops'; heated SMILE-G3 4.8/9.6 kWh modules introduced.

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