Key formula
Power is not the same as energy
Power in watts (W) indicates the rate at which a device uses energy. Energy consumption is commonly measured in kilowatt-hours (kWh).
Divide watts by 1000 to get kilowatts, then multiply by operating hours. When a device draws variable power, this simple method gives an estimate rather than a measurement.
Electricity consumption formula
Multiply power in kilowatts by hours used each day and the number of days. To estimate cost, multiply kWh by your local price per kWh.
Worked example: a fan
Consider a 60 W fan running 8 hours per day for 30 days. Its power is 0.06 kW, so estimated consumption is 0.06 × 8 × 30 = 14.4 kWh.
At an example rate of 0.20 USD per kWh, the estimated energy cost is 2.88 USD. Actual bills can include fixed fees, taxes, time-of-use rates and other charges.
When the estimate changes
Refrigerators, air conditioners and pumps often cycle on and off, so rated power is not necessarily average power.
For important decisions, use measured energy data, manufacturer specifications and your actual tariff structure.
Educational content: verify the conditions of your real situation before relying on the result.