How to use it
- Get a daily kWh figure for each appliance. The best source is a plug-in energy meter left in place for a full day.
- No meter? Use the yearly kWh on the energy label and divide by 365. A label showing 438 kWh a year works out to 1.2 kWh a day.
- Set the outage length you want to cover without recharging.
- Keep the usable share at 85 to 90% unless you know your unit's real efficiency.
The formula
battery (Wh) = (fridge + freezer kWh per day) × 1,000 × hours ÷ 24 ÷ usable share
With the starting numbers, 1.8 kWh a day for 24 hours at 90% usable gives 1,800 ÷ 0.90 = 2,000 Wh. A 48-hour outage doubles it to 4,000 Wh.
Reading the result
| Line | What it tells you |
|---|---|
| Battery target | The rated watt-hours to shop for, rounded up to the next 50 Wh. |
| Appliance energy | What the fridge and freezer will actually draw over the outage, before losses. |
| Combined average load | The steady watts to enter in the battery runtime calculator or the load builder. |
You may not need a battery straight away
Food-safety agencies in the US and Canada give the same rule of thumb: a fridge keeps food safe for about 4 hours if the door stays shut, a full freezer for about 48 hours, and a half-full freezer for about 24 hours. For a short outage, keeping the doors closed costs nothing. A battery matters once the outage runs past those windows.
Limits
- Energy labels use standard test conditions. A warm kitchen, frequent door openings or an older appliance will use more.
- Compressors draw a brief surge when they start, often several times their running watts. Capacity in watt-hours says nothing about that.
- Solar recharging is not included.
Common questions
Where is the yearly kWh figure?
On the yellow EnergyGuide label in the United States and the EnerGuide label in Canada. If the label is gone, search the model number on the manufacturer's site.
Can I run the fridge only part of the time?
Yes. Many people power the fridge and freezer for an hour or two every few hours and keep the doors shut in between. Use a fridge thermometer and keep it at or below 40°F (4°C).
Is a chest freezer easier to back up than an upright?
Usually. Cold air stays in a chest freezer when the lid opens, and they tend to use less energy for the same volume. Check your own label to be sure.
What surge rating do I need?
Check the appliance's manual for starting watts or locked-rotor amps. If you cannot find it, the generator sizing calculator shows how starting surge adds to running load.
Related
Reviewed October 2026. Results are planning estimates. Check them against your equipment's specifications. Report a problem.