Appliance duty cycle: estimate electricity cost when a device cycles on and off

How to calculate it

A nameplate wattage is a power rating, not automatically constant consumption. A simple model is energy in kWh = running watts × hours × duty fraction ÷ 1,000. Duty fraction is the time the load draws that running power divided by total observation time. Add other power states separately, such as standby watts when the compressor is off. For the best estimate, measure the device over representative days with a suitable energy meter; startup surges and variable-speed equipment are not captured by one watt value.

Worked example

Consider a 150 W refrigerator compressor running 40% of the day, with 5 W for controls during the other 60%. Compressor energy is 150 × 24 × 0.4 ÷ 1,000 = 1.44 kWh per day. Controls add 5 × 24 × 0.6 ÷ 1,000 = 0.072 kWh, totaling 1.512 kWh per day. For 30 days, that is 45.36 kWh; multiply by your tariff per kWh for the energy-only estimate. Do not claim this is a typical refrigerator consumption.

What can change the result

Ambient temperature, thermostat setting, loading, door openings, maintenance and season change the duty cycle. Tariffs may have tiers, demand charges or fixed fees. Some devices draw a different wattage while active; measure rather than forcing a single percentage. Annualizing one unusual hot day can mislead.

Use the calculators and keep reading

Put measured or realistic average watts and hours into the electricity calculator. Compare a 30% and a 60% duty case, then check the linked watts-to-kWh guide for units and billing assumptions. Keep fixed account charges outside the device’s incremental energy cost.

Try the electricity, electrical power calculator and read this related article for the context and assumptions.