How many hours will a battery back up your home?
Enter your battery size and load to get honest run-time — with depth-of-discharge and inverter losses built in — or work backwards to the battery size a target backup needs.
The formula
Runtime is usable energy divided by load. The honest part is usable — you never get the full nameplate to your appliances.
The two deratings are real physics, not padding. The usable fraction reflects the BMS depth of discharge limit — you don't drain a pack to 0%. The efficiency term is inverter and round-trip efficiency loss on the way out. Both defaults are editable above.
What runtime really depends on
- Nameplate kWh
- The energy printed on the datasheet. You only ever access a usable slice of it.
- Continuous load (kW)
- How fast you draw power. Halving the load roughly doubles the runtime, so outages shed to essentials.
- Usable fraction
- The depth-of-discharge window the BMS allows. LFP home batteries typically expose about 90%.
- Round-trip efficiency
- Energy lost in the inverter and conversion. Around 90–92% reaches the AC side.
Not modelled here, but real: battery aging trims usable capacity year over year, and cold temperatures cut both capacity and efficiency. Size with headroom rather than to the exact number.
Backup runtime reference
Honest run-times at the calculator defaults (usable 0.9, efficiency 0.92).
| Scenario | Battery | Load | Run-time |
|---|---|---|---|
| Fridge only, small battery | 5 kWh | 0.2 kW | 20.7 h |
| Essentials circuit | 10 kWh | 1 kW | 8.3 h |
| Partial home | 15 kWh | 3 kW | 4.1 h |
| Whole home | 20 kWh | 6 kW | 2.8 h |
FAQ
How long will my battery last in a power outage?
Divide usable energy by your continuous load. A 10 kWh battery at 90% usable and 92% efficiency delivers about 8.3 kWh, so a 1 kW essentials load runs roughly 8 hours. A larger whole-home load empties it much faster.
Why is real runtime less than battery kWh divided by load?
Two honest deratings. The BMS keeps a depth-of-discharge reserve so you never hit 0% (usable fraction, ~0.90), and the inverter loses energy converting DC to AC (round-trip efficiency, ~0.92). Multiply both by nameplate to get the usable kWh that actually reaches your load.
What load should I enter?
Use your continuous draw, not peak. Fridge-only is around 0.2 kW, an essentials circuit (lights, fridge, router, a few outlets) around 1 kW, a partial home 3 kW, and a whole home 6 kW or more. During an outage most homes shed load down to essentials.
Does the calculator account for cold weather and battery aging?
No — it gives the honest as-new figure. A battery a few years old holds less, and cold temperatures further cut usable capacity and inverter efficiency. Treat the result as an upper bound and size with headroom.
Need the battery behind the numbers?
Hua Power builds high- and low-voltage residential LFP systems in the kWh range these numbers point to. Tell us the load and backup hours and we will spec a stackable pack that fits.