How Long Will a 100Ah LiFePO4 Battery Last?
Estimate runtime for a 100Ah lithium battery using voltage, load watts, inverter efficiency and reserve.
There is no single runtime for a 100Ah battery because amp-hours do not specify load. Convert the battery to watt-hours and divide usable energy by actual watts.
What matters
- A nominal 12.8V 100Ah battery is about 1,280Wh before reserve.
- AC loads lose some energy through the inverter.
- High discharge rates and temperature can affect real-world output.
- Manufacturer specifications should define usable limits.
How to size it
- Calculate nominal Wh from voltage × Ah.
- Apply your reserve and efficiency assumption.
- Divide usable Wh by the load in watts.
- Check that BMS current can support the load.
If usable delivered energy is 1,100Wh, a steady 110W load is roughly a ten-hour problem, not counting other system consumption.
Common buying mistakes
- Dividing amp-hours directly by AC watts.
- Assuming every 100Ah battery has identical nominal voltage and BMS limits.
- Ignoring inverter idle draw for long low-power runtimes.
Runtime is a watt-hour equation. Use the battery-runtime calculator with your actual load instead of relying on generic 100Ah claims.
Use the numbers before choosing a model
Start with your load list. The battery needs enough watt-hours for the desired runtime and enough continuous/surge watts to operate the appliances at the same time. Then check whether your recharge method can replace the energy you expect to consume.
For outage planning, we recommend keeping reserve capacity instead of designing a system that reaches 0% at the end of the forecasted outage.
Next step
Estimate battery runtime → or Build a critical-load energy budget →.