Formula
Kilowatt-hours = watts × hours per day × days ÷ 1,000Direct answer
Multiply stated watts by operating hours and days, then divide by 1,000 to express the result in kilowatt-hours (kWh). The U.S. Energy Information Administration defines one kilowatt-hour as one kilowatt used for one hour and notes that one kilowatt equals 1,000 watts.
Variables and formula
- Appliance power: watts on the appliance label or from a measurement.
- Hours used per day: average operating hours in one day.
- Number of days: days using that operating pattern.
Kilowatt-hours = watts × hours per day × days ÷ 1,000
Reproducible example
An appliance rated at 1,200 W that runs 2 hours per day for 30 days uses 1,200 × 2 × 30 = 72,000 Wh. Dividing by 1,000 gives 72 kWh.
Interpretation
The result estimates energy from the power and operating time you enter. It is not a bill estimate: tariffs, fixed charges, demand charges and taxes are not included. Use the Electricity Cost Calculator only after entering a rate from the relevant bill.
Assumptions and limitations
- The entered wattage stays constant while the appliance runs.
- Daily use is representative of every entered day.
- Standby use, cycling, power factor, voltage changes and measured consumption are excluded.
Common mistakes
- Confusing watts (power) with watt-hours or kWh (energy over time).
- Entering minutes as hours.
- Applying a price per kWh before calculating the kWh amount.
Assumptions
- The stated power is constant while the appliance is operating.
- Hours per day is an average over the entered number of days.
- Standby load, cycling, power factor, voltage variation and electricity cost are excluded.
Sources and methodology
CalcMotive publishes the formula and assumptions so you can decide whether the estimate fits your use case. See our methodology standards.
- Measuring electricity; U.S. Energy Information Administration; accessed Aug 27, 2026