Battery Backup Runtime Calculator: Critical Loads, Usable kWh, and Inverter Losses
Last reviewed August 2, 2026. Confirm usable-capacity definitions, reserve behavior, discharge limits, and inverter ratings in the exact equipment manuals.
Direct answer: Runtime is usable AC energy divided by average AC load, including inverter idle consumption. Power compatibility is checked separately because enough kWh does not prove an inverter can start a pump or compressor.
Inputs
Runtime and power checks
Results appear here after you click Calculate. Enter your values, then compare the result with the interpretation notes below.
Interpretation
Energy-limited runtime estimates duration at the entered average load. The continuous and surge checks compare AC load with inverter ratings; they do not verify battery/BMS discharge-current limits, surge duration, motor starting waveform, transfer equipment, or branch-circuit design.
Formula or method
For nameplate mode, usable DC kWh = quantity × capacity × allowed depth of discharge × (1 − additional reserve) × temperature-retained factor × age-retained factor. For manufacturer-usable mode, depth of discharge is not applied again. Usable AC kWh then applies inverter efficiency. Runtime = usable AC kWh ÷ [(continuous watts + intermittent watts × duty cycle + inverter idle watts) ÷ 1,000].
Assumptions and limitations
- Manufacturer-stated usable capacity may already reflect internal reserves; only add the explicit user-selected reserve you actually want.
- Temperature and age factors are user assumptions, not predictions.
- Actual loads cycle, and inverter efficiency varies with power.
- Confirm battery continuous/peak current, inverter surge duration, transfer equipment, ventilation, and manufacturer instructions.
Worked example
If 5 kWh is already the usable AC energy after reserve, temperature, age, and inverter-loss assumptions, a 520 W average load has an energy-limited runtime of about 9.6 hours. That does not prove the inverter or battery can start the largest surge load; use the separate power warning.
Sources
- U.S. EIA: Measuring electricity: supports the relationship among watts, kilowatts, kilowatthours, power, and energy.