Short answer: most refrigerators need roughly 1 to 2 kWh (1,000 to 2,000 watt-hours) per day, but your own number depends on the model, so measure it. Divide your power station's usable watt-hours by the fridge's daily watt-hours to see how many days it can run. A 1,000 Wh power station often covers only about 12 to 20 hours of an efficient fridge once conversion losses are counted.
Watts, watt-hours and why a fridge is tricky
Watts are how fast the compressor draws power. Watt-hours (Wh) are the energy used over time. A power station's size is given in Wh, so you need your fridge's Wh per day. A refrigerator stays plugged in but cycles on and off to hold temperature, so its nameplate watts overstate what it uses on average. The U.S. Department of Energy notes that fridges cycle and suggests plug-in meters because they work well for devices that do not run constantly (DOE).
Runtime (hours) = usable Wh ÷ average fridge watts
Step 1: Find your fridge's daily watt-hours
- Best: plug a plug-in watt meter into the wall, plug the fridge into it, and leave it for 24 hours. Read the kWh and multiply by 1,000 to get Wh.
- Good: use the yearly kWh on the yellow EnergyGuide label. Divide by 365, then multiply by 1,000.
- Rough: a label in watts is a maximum draw. DOE's rule of thumb for fridge runtime is to divide plugged-in time by three, which gives an estimate of the hours at full wattage.
Step 2: Count usable capacity
A 1,000 Wh battery does not deliver 1,000 Wh to your fridge. Inverter and conversion losses take some. This article assumes 85% usable, which is a planning assumption, not a measured figure. Check your power station's manual for its stated efficiency.
Worked example (with stated assumptions)
Suppose your EnergyGuide label says 450 kWh per year.
- 450 kWh ÷ 365 = 1.23 kWh per day = 1,233 Wh per day
- Average draw: 1,233 Wh ÷ 24 h = 51 W
- Usable energy in a 1,000 Wh power station at 85%: 850 Wh
- Runtime: 850 ÷ 51 = about 16.6 hours
The chart shows the same fridge on four common power station sizes under the same assumptions. Opening the door, a warm kitchen, or a freshly loaded fridge raises the draw, so treat the numbers as a best case.
Example runtime at 51 W average and 85% usable capacity.
| Power station | Usable (85%) | Runtime at 51 W average |
|---|---|---|
| 500 Wh | 425 Wh | 8.3 hours |
| 1,000 Wh | 850 Wh | 16.6 hours |
| 2,000 Wh | 1,700 Wh | 33.3 hours |
| 3,000 Wh | 2,550 Wh | 50 hours |
Check the surge rating too
A compressor needs a burst of power to start, often several times its running watts. Pick a power station whose surge (peak) rating comfortably exceeds the fridge's start-up draw. A plug-in meter that records peak watts can show you yours, and the fridge's manual may list its rated amps.
Food safety comes first
Keep the doors closed. According to FoodSafety.gov, a refrigerator keeps food safe for about 4 hours without power, a full freezer for about 48 hours and a half-full freezer for about 24 hours (FoodSafety.gov). A power station that restores cooling before those windows pass can protect your food, so start it early.
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