How long will a 13.5 kWh home battery last in an outage?
Short answer: With about 12 kWh delivered after inverter losses, a 13.5 kWh battery runs essential loads averaging 500 W (fridge, lights, internet, furnace blower) for about 24 hours, and an average U.S. home (about 1.2 kW) for about 10 hours. Central AC or electric heat drains it in a few hours.
Runtime by average load
13,500 Wh, 90% through the inverter, run to empty. The load is the average over the day, not the peak: a fridge's 200 W compressor runs about half the time, so it averages 100 W or less.
| What is running | Average load | Runtime |
|---|---|---|
| Fridge, lights, internet, phones | 300 W | 40.5 h |
| Essentials plus a gas furnace blower | 500 W | 24.3 h |
| Most of a house, no AC or electric heat | 1,000 W | 12.2 h |
| Average U.S. home | 1,230 W | 9.9 h |
| House with a window AC running | 2,000 W | 6.1 h |
| Central AC cycling on a hot afternoon | 3,000 W | 4.1 h |
For scale: the EIA puts the average U.S. home at 10,791 kWh a year (2022), about 29.6 kWh a day or 1.2 kW on average. That is more than two full 13.5 kWh batteries every day.
Energy and power are separate limits
A battery has two ratings. Capacity (kWh) decides how long. Power (kW) decides what can run at once: a battery rated 5 kW continuous can't start a 3-ton central AC (11,000 W or more for a second) without a soft starter, however full it is. Units rated 11.5 kW continuous can. Check both against your loads in the home battery calculator.
How to stretch it
- Back up a critical-loads panel instead of the whole house, so the dryer or water heater can't drain the battery.
- Set a backup reserve. If the battery also stores solar for everyday use, keep 20–30% reserved for outages.
- Add solar. With panels, the battery recharges on sunny days; a 7 kW array makes roughly 20–35 kWh on a clear day, enough to keep essentials running.
- Skip electric resistance heat. A 1,500 W space heater uses about an eighth of the battery every hour.
Size a battery for your outage
Safety. Home batteries connect through a transfer device installed by a licensed electrician, with a permit, and with utility approval if they work alongside solar.