Home Battery Backup Calculator
Estimate how many hours a home battery can power your loads during an outage from its usable capacity and your average power draw.
How to use this tool
- Enter the battery's usable capacity in kWh (from its spec sheet).
- Estimate your average backed-up load in watts during an outage.
- Read the backup runtime in hours.
- Check the runtime in days for multi-day outage planning.
- Lower the load (back up fewer circuits) to extend runtime.
Estimate how long a home battery can keep the lights on. Enter the battery's usable capacity and your average load to see the backup runtime in hours and days.
Formula
Runtime is usable energy divided by power draw, with a unit conversion from kWh to watt-hours:
Runtime (hours) = Usable capacity (kWh) × 1000 ÷ Load (W)
Runtime (days) = Runtime (hours) ÷ 24
How it works
A battery's backup runtime is simply how much energy it can deliver divided by how fast you draw it. Capacity is given in kilowatt-hours of usable energy; loads are usually quoted in watts. Converting kWh to watt-hours (multiply by 1,000) and dividing by the load in watts gives the runtime in hours.
The key word is 'usable.' Battery packs reserve a fraction of their nameplate capacity to protect cell life, so a 13.5 kWh nameplate unit might offer about 13.5 kWh usable while some others reserve more. Enter the usable figure from the spec sheet, not the gross rating, for an honest runtime.
Average load matters more than peak. During an outage you typically back up essentials — refrigerator, lights, internet, a few outlets — whose combined average draw is far below the sum of their peak ratings, because most cycle on and off. Inverter conversion losses (a few percent) and the battery's own standby draw will shorten real runtime slightly versus this ideal estimate.
Worked example
A 10 kWh usable battery powering a 500 W load
- Convert capacity to watt-hours: 10 kWh × 1000 = 10,000 Wh.
- Runtime = 10,000 Wh ÷ 500 W = 20.00 hours.
- In days: 20.00 h ÷ 24 = 0.83 days.
Backup runtime 20.00 hours | Backup runtime 0.83 days
Backup runtime (hours) by usable capacity and continuous load
| Usable capacity | At 300 W | At 500 W | At 1000 W |
|---|---|---|---|
| 5 kWh | 16.7 h | 10.0 h | 5.0 h |
| 10 kWh | 33.3 h | 20.0 h | 10.0 h |
| 13.5 kWh | 45.0 h | 27.0 h | 13.5 h |
| 20 kWh | 66.7 h | 40.0 h | 20.0 h |
Key terms
- Usable capacity
- The energy a battery can actually deliver, after the reserve held back to protect the cells — lower than the nameplate capacity.
- Continuous (average) load
- The steady power your backed-up circuits draw over time, in watts — distinct from the brief surge when motors start.
- Depth of discharge (DoD)
- How much of a battery's capacity is used before recharging. Higher usable capacity reflects a higher allowed depth of discharge.
- Inverter
- The device that converts the battery's DC power to household AC. Its efficiency (typ. 90–95%) slightly reduces deliverable energy.
Frequently asked questions
- How long will a 10 kWh battery last?
- It depends entirely on your load. Backing up a 500 W essentials load (fridge, lights, Wi-Fi), a 10 kWh usable battery lasts about 20 hours. A 1,000 W load halves that to 10 hours.
- Should I use nameplate or usable capacity?
- Use usable capacity. Batteries hold back a reserve to protect the cells, so the deliverable energy is less than the nameplate rating. The spec sheet usually lists both.
- What's a typical essentials load during an outage?
- A refrigerator, LED lighting, phone/laptop charging, and internet equipment often average 300–600 watts combined, since the fridge compressor only runs part of the time. Adding well pumps, AC, or electric heat raises it sharply.