Watts measure power: the rate at which energy is used or supplied. Kilowatt-hours measure an amount of energy. A kilowatt-hour is not a more technical spelling of a kilowatt.
The calculation in one example
Suppose a device uses a constant 500 watts for two hours. That is 0.5 kilowatts multiplied by two hours, or one kilowatt-hour of energy. This is an illustrative calculation, not a measured claim about a particular appliance.
| Quantity | Unit | What it tells you |
|---|---|---|
| Power | W or kW | Rate at a moment or over a stated operating condition |
| Time | Hours | How long that condition lasts |
| Energy | Wh or kWh | Total use over the interval |
For constant power, energy in kWh equals watts divided by 1,000, multiplied by hours.
Why a nameplate is not the whole bill
Many appliances cycle on and off or use different power in different modes. Multiplying maximum rated power by every hour of ownership can overstate consumption. A measured energy total or a suitable standardized estimate may answer a different question from the rating plate.
To estimate the energy portion of a bill, multiply kWh by the applicable price per kWh. Label the assumptions. Actual bills may include fixed charges, taxes, time-dependent rates, or other components, so this calculation is not necessarily the final amount due.
Keep comparisons consistent
Compare devices performing a similar task over a similar period. A lower instantaneous wattage does not guarantee lower total energy if the task takes longer. The key is to identify whether a number describes speed of use or accumulated use before deciding what it says about efficiency or cost.
Sources
- US EIA: Measuring electricity
Watts measure power and watt-hours measure energy over time.