Short answer: divide the battery’s usable kWh by how many kW your home draws. A 13.5 kWh battery like the Tesla Powerwall 3 would run an average U.S. home (about 28 kWh a day) for under 12 hours, but essentials only, at around 0.5 kW, for about a day. With solar recharging it on sunny days, essentials can run for many days.
The formula
Hours = usable battery kWh รท average load in kW
Two limits matter:
- Capacity (kWh) decides how long. The Powerwall 3 stores 13.5 kWh (EnergySage).
- Power (kW) decides what can run at once. The Powerwall 3 delivers up to 11.5 kW continuously, enough for most household loads but not everything at the same time.
Runtime examples for 13.5 kWh
| Scenario | Average load | Runtime |
|---|---|---|
| Bare essentials: fridge, lights, Wi-Fi, phones | about 0.3 kW | about 45 hours |
| Essentials plus gas furnace fan and TV | about 0.6 kW | about 22 hours |
| Average whole home (865 kWh/month) | about 1.2 kW | about 11 hours |
| Whole home with central AC on a hot day | 3+ kW | under 5 hours |
Average whole-home use from the EIA (865 kWh per month in 2024). Load figures for the other rows are illustrative; measure yours from smart meter data. Real runtimes are a bit shorter because of inverter losses and any reserve the battery keeps.
What different appliances cost you in battery time
- Refrigerator: about 1.2-1.5 kWh a day for a standard model (PowerOutage.us), roughly a tenth of a 13.5 kWh battery.
- Electric heat, water heating and AC are the battery killers. Many backup setups exclude them.
- Well and sump pumps draw a lot briefly when they start; check the battery’s surge rating.
How to stretch it
- Back up a critical-loads panel instead of the whole house, so big loads can’t drain it. See whole-home vs partial backup.
- Keep a reserve. Set the battery to hold 20-30% for outages if you also use it for daily savings.
- Add solar. Even a winter day of sun can refill a good part of the battery.
- Add a second battery if outages last days and you don’t have solar.
- Cut loads during the outage: skip the dryer, oven and electric heater.
Batteries vs other backup
For multi-day outages without solar, a generator or a battery-plus-generator combo is often more practical. Compare them in home battery vs generator, and see whether a battery pays off financially in are home batteries worth it.
FAQ
Does the battery switch over automatically? Yes, with the right gateway or transfer equipment, usually within a fraction of a second.
Can solar charge the battery during an outage? Yes, if the system is designed for backup. Solar-only systems without a battery shut down when the grid goes down.