Method 1: a plug-in energy meter
A plug-in meter sits between the wall outlet and the appliance and records energy over time. It is the most accurate option for anything that plugs into a standard outlet.
- Check that the meter is rated for the appliance's current.
- Plug the meter into the wall and the appliance into the meter.
- Leave it for at least 24 hours. Several ordinary days are better, since they capture defrost cycles and door openings.
- Write down the total kWh and the hours elapsed.
- Convert to a daily figure: kWh ÷ hours × 24.
For example, 2.4 kWh over 48 hours is 1.2 kWh a day. To turn that into average watts, multiply by 1,000 and divide by 24: 50 W.
While the meter is connected, also note the highest watts you see when the compressor is running. That is the running figure for the generator calculator.
Method 2: the energy label
Large appliances carry a yearly energy figure: the yellow EnergyGuide label in the United States and the EnerGuide label in Canada. Both give kWh per year under standard test conditions.
daily kWh = yearly kWh ÷ 365
A label showing 438 kWh a year gives 1.2 kWh a day. Treat it as a starting point. A hot kitchen, a full household or an older appliance will use more.
Method 3: the nameplate
Every appliance has a plate or sticker listing watts, or volts and amps. Multiply volts by amps for a rough upper limit on its draw. This is the least accurate of the three, because the nameplate shows the maximum and many devices use much less most of the time.
Which number goes where
| Calculator | What to enter | Best source |
|---|---|---|
| Fridge and freezer backup | Daily kWh for each appliance | Meter, or label ÷ 365 |
| Battery runtime | Average watts across the day | Daily kWh × 1,000 ÷ 24 |
| Outage load builder | Watts and hours per day; for cycling appliances, average watts and 24 hours | Meter |
| Generator sizing | Running watts, plus extra starting watts for motors | Meter for running; manual for starting |
A common mistake with fridges
Reading 180 W on the meter while the compressor runs and multiplying by 24 gives 4,320 Wh a day. The same fridge measured over a full day might use 1,200 Wh. The first figure would have you buying a battery more than three times too large. The reverse mistake is just as real: sizing an inverter from the 50 W average will leave it unable to start the compressor.
Things a plug-in meter cannot measure
- Hard-wired equipment such as a furnace, well pump or water heater. Use the nameplate and the manual, or ask an electrician to take a clamp-meter reading.
- 240 V appliances. Standard plug-in meters are for 120 V outlets.
- Start-up surge. Most consumer meters update too slowly to catch it. Use the manufacturer's starting watts or locked-rotor amps.
Medical equipment
For a CPAP machine, oxygen concentrator or any other medical device, follow the manufacturer's backup-power instructions and ask your equipment supplier what they recommend. Heated humidifiers can more than double a CPAP machine's draw, and some devices need a pure sine wave supply.
Sources
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Reviewed October 2026 by Lee Palmer. This guide is general information, not professional advice. Suggest a correction.