How long will a home battery last in a power cut?
A full 13.5 kWh battery, the size of a Powerwall, runs the fridge-freezer, the heating controls, the broadband, the lights and the phones for about 4 days 12 h in a typical British home, and about 2 days 7 h in an American one with a gas furnace. Starting at a 20% reserve, the British home gets about 21 h 39 min.
Choose a battery or type its usable size, say how full it is and what you want to keep running. Add solar only if your system keeps it working in a power cut (many do not).
Every size at a glance
How long a full battery lasts, by what you keep running. Click a size for the half-full and reserve figures, single appliances and solar.
| Usable size | Fridge, internet, lights | Essentials with heating | Plus cooking and TV | Whole house as normal |
|---|---|---|---|---|
| 5 kWh | 2 days 1 h | 40 h 5 min | 25 h 51 min | 14 h 54 min |
| 10 kWh | 4 days 2 h | 3 days 8 h | 2 days 4 h | 29 h 48 min |
| 13.5 kWh | 5 days 12 h | 4 days 12 h | 2 days 22 h | 40 h 14 min |
| 15 kWh | 6 days 3 h | 5 days | 3 days 6 h | 44 h 43 min |
| 20 kWh | 8 days 4 h | 6 days 16 h | 4 days 7 h | 2 days 12 h |
| 27 kWh | 11 days | 9 days | 5 days 20 h | 3 days 8 h |
| Usable size | Fridge, internet, lights | Essentials with heating | Plus cooking and TV | Whole house as normal |
|---|---|---|---|---|
| 5 kWh | 44 h 8 min | 20 h 18 min | 16 h 43 min | 3 h 43 min |
| 10 kWh | 3 days 16 h | 40 h 36 min | 33 h 26 min | 7 h 27 min |
| 13.5 kWh | 4 days 23 h | 2 days 7 h | 45 h 9 min | 10 h 4 min |
| 15 kWh | 5 days 12 h | 2 days 13 h | 2 days 2 h | 11 h 10 min |
| 20 kWh | 7 days 9 h | 3 days 9 h | 2 days 19 h | 14 h 54 min |
| 27 kWh | 9 days 22 h | 4 days 14 h | 3 days 18 h | 20 h 7 min |
The essentials use about 2.1 kWh a day in a British home and 4.8 kWh in an American one, where the furnace blower is the big draw. Covering them for a day needs about 3 kWh of usable battery (UK) or 5.9 kWh (US); three days needs about 9 kWh or 17.7 kWh.
Named batteries
- Tesla Powerwall 313.5 kWh · 11 kW · UK and US
- Tesla Powerwall 213.5 kWh · 5 kW · UK and US
- GivEnergy All in One13.5 kWh · 6 kW · UK
- GivEnergy Giv-Bat 9.59.5 kWh · UK
- EcoFlow PowerOcean4.8 kWh · 6 kW · UK
- Anker SOLIX X15 kWh · 6 kW · US
- Enphase IQ Battery 5P5 kWh · 3.2 kW · UK and US
- Enphase IQ Battery 10C10 kWh · 7.1 kW · US
- FranklinWH aPower 215 kWh · 10 kW · US
- SolarEdge Home Battery9.7 kWh · 5 kW · UK and US
- Sigenergy SigenStor8.76 kWh · 4 kW · UK
- Fox ESS EP1210.37 kWh · UK
How full is it when the power goes?
A home battery spends most days charging and discharging to save money, so a power cut finds it at whatever level it happens to be. In the evening in winter that can be close to empty. Most systems let you set a backup reserve that the battery will not use on ordinary days (Tesla calls it Backup Reserve; GivEnergy's app has a similar minimum charge for its emergency supply). The tables here show a full battery, a half-full one, and one sitting at a 20% reserve.
If a storm is forecast, charge it to full. Tesla's Storm Watch does this automatically when a weather warning is issued for your area; on other systems you raise the reserve or force a charge in the app.
How the sum works
Runtime is the energy in the battery when the cut starts, times what gets through the inverter, divided by what the house draws on average plus what the battery system draws itself.
- Usable size. The maker's usable figure, not the bigger nominal one.
- Through the inverter. 90% cautious, 94% typical, 97% best. These are round figures; makers rarely publish a one-way number.
- The battery system's own draw. The inverter, the battery management and the gateway, about 15 to 50 W (30 W typical) unless the maker publishes a figure.
- What the house draws. Each appliance's watts × the share of the time it runs × hours a day, from the same appliance catalogue as the rest of the site. Lights are six LED bulbs for six hours. "The whole house as normal" is Ofgem's typical use for a medium household, 2,500 kWh a year from July 2026 (Ofgem) or the EIA average for a US home, 10,791 kWh a year (2022) (EIA), spread evenly over the day.
- Solar. Panel size × peak sun hours for the month and region × 75% for real-world losses. If a day's sun covers a day's use and the battery holds 60% of a day's use for the night, the house keeps going. Averages hide dull weeks, so treat "keeps going" as "for as long as the weather holds".
Big loads matter as well as energy. A battery can only supply its continuous output at once (often 5 to 11 kW), so a kettle, an oven and a heater together can trip it even when it is full. The named battery pages check this.
No home battery? A power station for the essentials
A portable power station does the same job for the things that plug in, without an installer. The biggest ones hold a third to a half of a home battery. Here is how long each runs the fridge-freezer, the heating, the broadband, the lights and the phones, starting full:
| Power station | Capacity | UK home | US home |
|---|---|---|---|
| Zendure SuperBase V6400 | 6,438 Wh | 40 h 34 min | 19 h 50 min |
| EcoFlow DELTA Pro Ultra | 6,144 Wh | 39 h 47 min | 19 h 43 min |
| Jackery Explorer 5000 Plus | 5,040 Wh | 34 h 48 min | 16 h 41 min |
| Zendure SuperBase V4600 | 4,608 Wh | 31 h 13 min | 15 h 7 min |
| EcoFlow DELTA Pro 3 | 4,096 Wh | 27 h 45 min | 13 h 26 min |
The boiler or furnace is the catch: it is usually wired in, not plugged in, so a power station can only run it through a socket or a changeover switch an electrician fits. The boiler guide and the furnace guide explain how. Each station page has the full numbers and where to buy it.
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Useful whatever you have
A battery that is not wired for backup, or one that runs flat on day two, still leaves you needing light, phone charging and a way to keep food cold. The kits cover the cheap things that matter most.
Plug-in power failure lights. They live in a socket as a night light, charge themselves, and switch on the moment the mains goes off, so you are never hunting for a torch in the dark.
Look for: A built-in battery you can pull out and carry as a torch, and a UK three-pin plug if you are in the UK.
Power bank, 20,000 mAh. Keeps a phone going for days if you use it sparingly.
Look for: Capacity in watt-hours as well as mAh (about 74 Wh for 20,000 mAh), USB-C in and out, and keep it charged.
Mini UPS for the router. Home phones now run through the broadband router, so when it goes off in a power cut the landline goes with it. A small battery box keeps the router, and the phone plugged into it, working.
Look for: DC outputs that match the voltage printed on your router's power adapter (usually 12 V), enough of them for the fibre box too if you have one, and a capacity in watt-hours.
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