Watts to kWh: How to Calculate Electricity Use
Watts tell you the power a device is drawing. Kilowatt-hours tell you how much energy it uses over time. You need both power and time to estimate a bill.
Convert watts into energy
Energy in kWh = watts × hours ÷ 1,000
The division by 1,000 converts watt-hours to kilowatt-hours. For a constant 50 W load over a full day, 50 × 24 ÷ 1,000 = 1.2 kWh. The same device running for 8 hours uses 0.4 kWh.
| Power | Time | Energy |
|---|---|---|
| 10 W | 1 hour | 0.01 kWh |
| 100 W | 8 hours | 0.8 kWh |
| 50 W | 24 hours | 1.2 kWh |
| 200 W | 24 hours | 4.8 kWh |
Turn kWh into cost
Cost = energy in kWh × price per kWh
At an example price of 18¢/kWh, 1.2 kWh costs $0.216, or about 22¢. Keep the unrounded number when extending the estimate: $0.216 × 365 = $78.84 a year. Rounding the daily result first would introduce an avoidable error.
The calculator accepts cents per kWh. Enter 18 for 18¢, not 0.18. Use your bill to find the right rate.
When the power changes
Break the day into periods, calculate each period’s energy, and add them. A device drawing 100 W for 3 hours and 20 W for 21 hours uses (100 × 3 + 20 × 21) ÷ 1,000 = 0.72 kWh a day.
Alternatively, use a measured average over the full period. If a meter records 0.72 kWh in 24 hours, the average is 30 W. Do not multiply a peak reading by 24 unless you intend to model that peak continuing all day.
Three unit checks before calculating
- W and kW differ by 1,000. 100 W is 0.1 kW.
- kWh already includes time. If a meter reports total kWh for a week, multiply that total by the rate; do not multiply by the week’s hours again.
- A power rating is not a measured daily total. Use an actual watt reading or a recorded kWh total for your workload when available.
Measure energy over a representative period before extending it into a yearly estimate.
Sources and calculation notes
Worked examples are Always On Cost calculations using the stated assumptions, not product tests or utility quotes. Unless noted otherwise, estimates use a fixed rate and repeating daily schedule, with 365 days per year. USD amounts are rounded for display.