Transparent calculation
Formula and units
Power describes the rate of electricity use. Energy adds time: one kilowatt used for one hour equals one kilowatthour (kWh).
Power in kW = watts ÷ 1,000
Active-day energy = power in kW × hours
Monthly energy = active-day kWh × active days
Cost = energy in kWh × USD price per kWh
Annual estimate = monthly estimate × 12
Worked example
For a 1,500 W device used 3 hours on 30 days each month at $0.16/kWh:
- Active day
- 4.5 kWh · $0.72
- Month
- 135 kWh · $21.60
- Year
- 1,620 kWh · $259.20
Electricity cost questions
Check the product label, nameplate, manual, manufacturer website, or a plug-in electricity meter. If an appliance cycles on and off, a measured average power value will usually represent real use better than its maximum rating.