Practical guide
Fridge battery runtime: measure watts and duty cycle
A fridge does not usually draw its nameplate watts continuously. The useful starting point is energy over time, plus a separate startup-power check.
Measure the energy first
Use an appropriate energy meter according to its instructions, and observe a representative period with normal door openings and ambient conditions. Enter measured average watts with a 100% duty cycle. Alternatively, enter running watts with the fraction of time the compressor is on. Do not apply a duty-cycle reduction to an already averaged reading.
A transparent example
Assume a fridge averages 35 W over a full day. Its appliance energy is 35 × 24 = 840 Wh. At an assumed 88% conversion efficiency, plus a 5 W station standby draw for 24 hours, battery draw is 840 ÷ 0.88 + 120 = 1,074.55 Wh. Holding 15% of the battery in reserve gives a nominal capacity requirement of 1,074.55 ÷ 0.85 = 1,264.18 Wh.
Those are illustrative assumptions, not a measured fridge or power station. Temperature, defrost cycles, battery condition and real inverter behaviour can change the outcome.
Check startup separately
Battery capacity in Wh is not an output-power rating. Check the fridge’s actual startup demand and duration against the station’s surge specification, as well as running power. Do not infer a fixed surge multiplier for every fridge.
Reference: OUPES runtime calculation and limitations.