Working out what it costs to run an appliance comes down to one multiplication, but two details change the answer a lot: whether the appliance really runs at full power, and what you actually pay per kWh.
The basic formula
Power is measured in watts (W) and energy in kilowatt-hours (kWh). A 1,000 W appliance running for one hour uses 1 kWh.
Cost = energy (kWh) × price per kWh
Example
A 2,000 W oven used for 45 minutes (0.75 h): 2,000 × 0.75 ÷ 1,000 = 1.5 kWh. At $0.15 per kWh each use costs 1.5 × 0.15 = $0.225. Three times a week for a year (156 uses): about $35.10.
Appliances that don’t run at full power
Fridges, air conditioners, thermostat-controlled heaters and ovens switch their element or compressor on and off to hold a temperature. Their rated power is a maximum. That is why our calculators include a load factor or share of time at full power: multiply the power by that percentage before applying the formula.
For appliances with an energy label, the most reliable figure is usually the annual (kWh/year) or per-cycle consumption printed on the label, measured with a standard test method.
Getting real data
- Rating plate: shows maximum power in W or kW.
- Energy label (EU label or US EnergyGuide): annual or per-cycle consumption under test conditions.
- Plug-in energy meter: the most accurate option; it measures real energy over several days.
Which price per kWh to use
The unit rate in your contract excludes fixed charges and some taxes. To find your average all-in price, divide a bill’s total by the kWh used; our electricity bill estimator does it for you. To cost one more appliance, use the variable price (energy plus proportional taxes), because you pay fixed charges anyway.
Standby consumption
Many devices draw power even when “off”. Multiply standby power by the hours spent that way: 2 W for 20 h a day is 0.04 kWh daily, about 14.6 kWh a year.