If the Power Goes Out at Night, How Many Hours Will a Home Battery Last?
Summary
Runtime is the energy left when the outage starts divided by the load, and at night the energy left is mostly set by how much you told the battery to hold back. On Tesla's Powerwall 3 that setting is Backup Reserve. With 13.5 kWh and a 20% reserve, a nighttime outage starts from 2.7 kWh, which covers a 400 W average load for just under seven hours; at 50%, the same load runs for about 17.
What Backup Reserve holds back
Tesla describes Backup Reserve as the share of Powerwall's stored energy that is automatically saved for backup power, set with a slider in the Tesla app. Setting it to 100% rules out Self-Powered and Savings modes, and anything below 20% brings up a "Low Backup Reserve" indicator. Through an evening of running the house from the battery, the charge falls to that reserve, so a midnight outage usually begins right there. Once the outage is underway, Powerwall may discharge below the reserve.
| Reserve setting | Energy at start | 400 W average load | 1 kW average load |
|---|---|---|---|
| 20% | 2.7 kWh | about 6.8 h | about 2.7 h |
| 50% | 6.75 kWh | about 17 h | about 6.8 h |
| 100% | 13.5 kWh | about 34 h | about 13.5 h |
The table divides Powerwall 3's 13.5 kWh by the load and ignores conversion losses. That 13.5 kWh is the datasheet's nominal figure, given for 25°C (77°F), beginning of life, and 3.3 kW charge and discharge.
An outage that hits a nearly empty battery
If stored energy is below 5% when the grid fails, Tesla says the home loses power immediately; Powerwall then holds what is left and waits for solar to recharge it the next morning. Enphase builds a similar floor into IQ Battery 5P, whose 5.0 kWh usable rating includes a 2% safety-critical limit kept for long outages, with another 3% held for the battery electronics overnight. When grid power returns, Powerwall charges back above the reserve before resuming normal operation.
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Sources
- Tesla — Backup Reserve (Powerwall Support)
- Tesla — Powerwall 3 Datasheet (2025)
- Enphase — IQ Battery 5P Data Sheet (DSH-00010-11.0, 2025-05-06)
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