ELECTRICITY, EXPLAINED
kW vs. kWh: what is the difference?
BillMellow editorial · Updated October 25, 2026
kW measures power. kWh measures energy over time. A device drawing 1 kW for one hour uses 1 kWh. A device drawing 2 kW for half an hour also uses 1 kWh. The two units answer different questions.
To estimate energy: watts ÷ 1,000 × operating hours = kWh. To estimate the variable energy cost: kWh × dollars per kWh.
Watts, kilowatts and kilowatt-hours
A watt is a unit of power. One kilowatt is 1,000 watts. A kilowatt-hour combines that power with time. It is not the same as “kilowatts per hour.” On a residential electric bill, the usage line commonly reports energy in kWh.
| Unit | What it tells you | Illustrative example |
|---|---|---|
| W | Power at a stated operating level | 1,500 W heater setting |
| kW | The same power in thousands of watts | 1,500 W = 1.5 kW |
| kWh | Energy used over a period | 1.5 kW × 4 hours = 6 kWh |
How to convert watts to kWh
- Find the device’s average operating power in watts.
- Divide watts by 1,000 to get kW.
- Multiply by the time it actually draws that power.
A 100-watt device operated at that level for 10 hours uses 100 ÷ 1,000 × 10 = 1 kWh. A 1,000-watt device operated for one hour uses the same energy. A higher power rating does not determine monthly cost without an operating schedule.
If the input is already measured energy, such as 2 kWh from a meter over a day, do not multiply it by operating hours again. You already have the energy for that period.
How to turn kWh into an electricity cost
Multiply energy by the applicable variable price. At an illustrative $0.18/kWh, 6 kWh costs $1.08 in variable energy charges. Repeating that use for 30 days costs $32.40. These calculations do not add a monthly customer charge or taxes.
Convert cents to dollars before entering a rate: 18 cents per kWh is $0.18/kWh. Entering 18 instead of 0.18 makes the answer 100 times larger. Our electricity cost calculator accepts dollars per kWh.
Dividing your entire bill by total kWh produces an all-in average cost. That may include fixed fees. It is useful for describing a past bill but can be a poor estimate of the cost of one extra hour of appliance use. Read how to separate electric bill charges.
Why nameplate watts × 24 hours can be misleading
Equipment may cycle, change output or draw a different amount of power in different modes. A stated maximum power is not necessarily an all-day average. Match the number to the operating condition you are estimating.
For variable equipment, use measured kWh over a representative period when possible. Otherwise, use a supported average power and the hours that apply to that average. The result should be labeled as an estimate. A sample wattage in a calculator is not a measurement of your appliance.
Two devices, the same energy
A 1,500-watt device drawing that power for two hours uses 3 kWh. A 500-watt device drawing that power for six hours also uses 3 kWh. At $0.18/kWh, either schedule costs $0.54 in energy. This compares the math, not whether the devices provide an equivalent service.
Use the space heater cost guide for a daily and monthly table. For a broader device comparison, see appliance electricity use.
Unit definitions: US EIA: measuring electricity. Numerical examples are illustrative calculations.
Keep exploring
- Why is my electric bill so high?
- How much does a 1,500-watt space heater cost to run?
- Electricity supply vs. delivery charges
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