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.
Runtime is only one constraint: make the load list with the critical-loads planner, test starting power with the inverter-size calculator, and review the complete backup integration guide.
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.