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Battery Backup Runtime Calculator

Backup runtime from usable energy, inverter efficiency, standby draw, and the load, plus the load the bank can sustain for a target outage length.

Inputs

Bank and load

Nameplate or usable kWh of the installed bank. If the datasheet already gives usable kWh, set depth of discharge to 100%.

Share of nameplate you cycle. Lithium 80-90%, lead-acid 50%.

DC-to-AC conversion. 85-95% typical for hybrid inverters.

Measured or estimated average watts during the outage, including cycling appliances averaged over time.

Inverter no-load consumption, 15-50 W typical. It dominates runtime for small loads.

The outage length you want to cover, used to report the load the bank can sustain.

Result

Runtime
0.33 days
Runtime at this load
7.9 hrs
Load the bank can carry for the target
993 W
Energy after inverter losses
8.10 kWh
Runtime ignoring standby draw
8.1 hrs
Load including standby
1,020 W
Usable energy
9.00 kWh

What to check

  • Average load in watts: measure a real outage or list essentials, then average cycling appliances over the hour rather than using peak watts.
  • Standby draw of the inverter from the datasheet; it can dominate runtime for refrigerator-only outages.
  • Usable kWh rather than nameplate, with the warranty depth of discharge applied.
  • Surge behavior: motors and compressors draw several times running watts at start, which the inverter must carry even when the energy budget is fine.
  • Whether loads are simultaneous; the average must reflect what runs at the same time, not the sum of nameplate ratings.

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Screening estimate using a constant average load. Real runtime varies with load shape, inverter idle draw under changing load, temperature, and battery age. It does not evaluate surge or motor starting; confirm inverter continuous and surge ratings, wiring, and protection against the manufacturer data and the locally adopted code.

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