Rated power is not average power
A label describes a device under defined conditions. A refrigerator or air conditioner cycles and may modulate power. Multiplying the maximum rating by 24 hours can substantially overestimate consumption.
Model a cycling load
Energy in kWh equals watts divided by 1,000, multiplied by hours, days and duty cycle as a fraction. A 150 W refrigerator over 24 hours for 30 days at an illustrative 35% duty cycle uses 37.8 kWh.
Choose measured or rated power
The duty cycle is an input, not a promise about a particular model. Use a plug meter where safe and appropriate, a manufacturer energy label, or a measured average. If you enter measured average watts, use a 100% duty cycle to avoid applying the adjustment twice.
Track changing conditions
For variable-speed equipment, compare several representative days. Weather, thermostat settings, door opening and room conditions can change the average. Record the assumption alongside the result.
Convert accumulated energy into average power
A meter recording 0.9 kWh over 20 hours gives 0.9 × 1,000 ÷ 20 = 45 W average. Enter that value at 100% duty cycle over the period being projected. A simple on/off factor can miss fan-only consumption; a full-interval energy reading captures all states inside the measurement boundary.
Appliance Energy Cost Calculator
Worked example: Use an average reading for a cycling refrigerator
Related reading: How to measure appliance energy across a full operating cycle
Reference: Virginia Tech Extension: estimating appliance energy use.