Calculator
Backup power calculator
A fridge doesn't run its compressor all the time, so backup needs depend on the duty cycle, not just the nameplate watts. Enter the numbers to size an inverter and battery bank.
W
%
Share of time it runs.
hours
V
%
~50% lead-acid, ~80–90% lithium.
%
- Battery capacity
- 59 Ah @ 24 V
- Energy needed
- 960 Wh
- Inverter (min.)
- 1500 VA
- Average draw
- 240 W
Inverter size allows ~3× running VA for compressor start-up; pure sine-wave inverters are recommended. Keep the door/lid shut during cuts — opening it raises the duty cycle.
How this is calculated
- Average power = running power × duty cycle (the share of time the compressor runs; 50–70% is common in hot weather).
- Energy needed = average power × hours of backup.
- Battery capacity (Ah) = energy needed ÷ (battery bank voltage × usable depth of discharge × inverter efficiency).
- Inverter VA must cover the compressor's starting surge — we suggest at least 3 × running VA. Pure sine-wave inverters are recommended for compressors.
Results are planning estimates, not a design. For a binding specification, share your site details and we'll check them.
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