All worked examples below use explicitly chosen inputs. They are not measured household averages, utility quotes, or guaranteed savings.
Watts are a rate; kWh is an amount
A watt measures power at a moment. A kilowatt is 1,000 watts. A kilowatt-hour measures accumulated energy: one kilowatt maintained for one hour. You need time as well as power to calculate energy.
A 500 W appliance used for two hours and a 1,000 W appliance used for one hour each consume 1 kWh at constant draw. That equality concerns electricity only, not how much useful work either appliance performs.
Reference: EIA: measuring electricity
Convert minutes before multiplying
Divide minutes by 60. Do not treat 30 minutes as 0.30 hours; it is 0.5 hours. The following table assumes a constant 1,500 W draw and an illustrative price of $0.20/kWh.
| Operating time | Hours | Energy | Cost |
|---|---|---|---|
| 10 minutes | 0.1667 | 0.25 kWh | $0.05 |
| 20 minutes | 0.3333 | 0.50 kWh | $0.10 |
| 30 minutes | 0.5 | 0.75 kWh | $0.15 |
| 60 minutes | 1 | 1.50 kWh | $0.30 |
Extend one session into a monthly estimate
For a 20-minute, 1,500 W session repeated twice daily for 30 days: 0.5 × 2 × 30 = 30 kWh. At $0.20/kWh, the monthly electricity cost is $6. Count the days the appliance is actually used.
If power varies, a nameplate-times-hours calculation is a scenario rather than a measured total. Use average watts across the session, or measured kWh. When average watts already include cycling, leave active time at 100%.
Three errors that change the answer
Check the units before you compare devices. A daily figure and an annual label figure describe different periods.
- Enter 20 cents or 0.20 dollars per kWh, not 20 dollars.
- Do not convert a refrigerator’s annual kWh into watts and then assume a second duty cycle.
- Input power is different from a power supply’s capacity or a microwave’s advertised cooking output.
Sources & calculation notes
References checked 2026-10-09. Example arithmetic is calculated by HomeWattCheck. Article review dates are separate from the EIA data period displayed in our calculators.
Each calculator states its scope beside the result. Appliance tools estimate electricity at one rate; the time-of-use comparator includes two price periods and entered fixed fees. Complex tariffs may need additional calculations. State averages describe reported residential revenue per kWh, not your utility’s offer. See our data sources and methodology or state rate comparison.