How long will a 13.5 kWh battery last in a power cut?
A full 13.5 kWh battery runs the essentials (fridge-freezer, heating controls, broadband, lights and phones) for about 4 days 12 h in a British home and 2 days 7 h in an American home with a gas furnace. Run the whole house as normal and it lasts about 40 h 14 min (UK) or 10 h 4 min (US).
A Powerwall or a GivEnergy All in One. Keep it to the fridge, the internet and the lights and it stretches to about 5 days 12 h. How full it is when the power goes matters as much as its size: see the half-full and reserve columns.
By what you keep running
| What it runs | Full | Half full | At a 20% reserve |
|---|---|---|---|
| Fridge, internet, lights and phones | 5 days 12 h | 2 days 18 h | 26 h 26 min |
| The essentials, with the heating | 4 days 12 h | 2 days 6 h | 21 h 39 min |
| Essentials, heating, cooking and TV | 2 days 22 h | 34 h 53 min | 13 h 58 min |
| The whole house, as normal | 40 h 14 min | 20 h 7 min | 8 h 3 min |
| What it runs | Full | Half full | At a 20% reserve |
|---|---|---|---|
| Fridge, internet, lights and phones | 4 days 23 h | 2 days 12 h | 23 h 50 min |
| The essentials, with the heating | 2 days 7 h | 27 h 25 min | 10 h 58 min |
| Essentials, heating, cooking and TV | 45 h 9 min | 22 h 34 min | 9 h 2 min |
| The whole house, as normal | 10 h 4 min | 5 h 2 min | 2 h 1 min |
Heating means the gas boiler's pump, fan and controls, on for ten hours a day. The gas does the heating; the battery only runs the electrics. Heating means a gas furnace's blower and controls, on for ten hours a day. The blower is the big draw; the gas does the heating.
One appliance on its own
For a single job, such as keeping a freezer cold, a 13.5 kWh battery goes a long way. Heaters and car charging are the opposite. Each runs all day and night here, cycling as it normally would.
| On its own | Average draw | Runs for |
|---|---|---|
| Fridge-freezer (full size) | 42 W | 7 days 8 h |
| Chest freezer | 40 W | 7 days 13 h |
| Gas combi boiler (its electrics) | 51 W | 6 days 13 h |
| Space heater, 1,500 W | 1,500 W | 8 h 17 min |
| Electric car (3-pin "granny" charger) | 2,300 W | 5 h 27 min |
| On its own | Average draw | Runs for |
|---|---|---|
| Fridge-freezer (full size) | 53 W | 6 days 10 h |
| Chest freezer | 48 W | 6 days 19 h |
| Gas furnace (blower and controls) | 300 W | 38 h 27 min |
| Sump pump | 240 W | 47 h |
| Well pump (1/2 hp) | 200 W | 2 days 7 h |
| Window air conditioner, 8,000 BTU | 490 W | 24 h 24 min |
| Space heater, 1,500 W | 1,500 W | 8 h 17 min |
With solar that works in a power cut
If the panels keep charging the battery in a cut (they need a system built for it), a sunny day tops it up. With 4 kWp of panels in the south of England, an average day's sun covers the essentials in every month except December. These are averages. A week of thick cloud, common in a British winter, can cut the panels to a fraction of this, so in the darker months plan on the battery alone and count the sun as a bonus.
| Month | Starting full |
|---|---|
| Jan | Keeps going |
| Feb | Keeps going |
| Mar | Keeps going |
| Apr | Keeps going |
| May | Keeps going |
| Jun | Keeps going |
| Jul | Keeps going |
| Aug | Keeps going |
| Sep | Keeps going |
| Oct | Keeps going |
| Nov | Keeps going |
| Dec | 22 days 18 h |
With 7 kW of panels in the US south-east, where the furnace is the big draw, an average day's sun covers the essentials in every month.
| Month | Starting full |
|---|---|
| Jan | Keeps going |
| Feb | Keeps going |
| Mar | Keeps going |
| Apr | Keeps going |
| May | Keeps going |
| Jun | Keeps going |
| Jul | Keeps going |
| Aug | Keeps going |
| Sep | Keeps going |
| Oct | Keeps going |
| Nov | Keeps going |
| Dec | Keeps going |
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.
Batteries this size
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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These are estimates from the site's model, not a maker's claim: stored energy × 94% through the inverter ÷ (what the house draws on average + about 30 W for the battery system itself). The sum is explained here.
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