How to convert watts to kWh and electricity cost
Divide watts by 1,000 and multiply by hours to get kWh. If watts are a full-power rating, also account for cycling. If watts are already a measured average, use them directly.
By HomeCost Radar · Reviewed 2026-09-27 · Editorial policy
Three ways to describe the same day
Assume a device rated at 1,000 W, used over four elapsed hours and drawing full power for half that period. The $0.20/kWh price is illustrative and is not a current state rate.
| Input method | Energy calculation | Daily cost at $0.20/kWh |
|---|---|---|
| Rated power plus cycling | 1,000 / 1,000 × 4 × 0.5 = 2 kWh | $0.40 |
| Measured average of 500 W | 500 / 1,000 × 4 = 2 kWh | $0.40 |
| Meter reports 2 kWh | Use the 2 kWh reading directly | $0.40 |
What the example shows: All three descriptions can represent the same energy. Multiplying the measured 500 W average by another 50% duty factor would incorrectly halve the estimate to 1 kWh.
Choose the right unit for the question
Watts describe power; kilowatt-hours describe energy accumulated over time. EIA defines one kilowatt as 1,000 watts and a kilowatt-hour as one kilowatt used for one hour. Keep the time period beside every energy total.
Convert minutes to hours first. A 1,200 W assumed constant load for 15 minutes is 1.2 × 0.25 = 0.300 kWh. Multiplying by 15 as though the minutes were hours would overstate energy by a factor of 60.
Count days and devices once
If the worked daily load runs on 22 days, monthly energy is 2 × 22 = 44 kWh and the illustrative cost is $8.80. For two identical devices used on that schedule, double both values. Do not multiply by quantity if the measured watts already cover both devices.
A 22-workday estimate should not silently become 30 workdays. To estimate a year, choose the annual number of comparable days explicitly; multiplying one month by 12 is only appropriate when that month represents the rest of the year.
Price energy with the rate that applies
At a flat variable rate, multiply kWh by dollars per kWh. Convert 20 cents to $0.20 before multiplying. Fixed fees do not belong in the watts-to-kWh conversion, and time-of-use plans require assigning energy to the relevant period.
When the only available number is maximum label power, present the result as a full-input scenario or disclose the assumed cycling fraction. A precise-looking dollar total cannot make an uncertain runtime accurate.
Frequently asked questions
Can I convert watts to kWh without hours?
Is a kWh the same as a unit of electricity?
Sources and assumptions
The worked examples are HomeCost Radar calculations from the stated inputs. Sources below support the concepts identified beside each link; they do not certify our assumed wattages or product savings.
- EIA: measuring electricity — The distinction between power in watts and energy in kilowatt-hours.