Watts vs kilowatt-hours: what's the difference?
Watts measure power (how fast energy is used); kilowatt-hours measure energy (how much is used over time). Clear examples, conversions and how to read your electricity bill.
- Watts (W) measure power: the rate at which an appliance uses energy right now.
- Kilowatt-hours (kWh) measure energy: power multiplied by time. 1 kWh = 1,000 W for one hour.
- Your bill charges per kWh, so a low-wattage device left on all the time can cost more than a powerful one used briefly.
The car analogy
Think of a car: speed (miles per hour) is how fast you’re going right now, and distance (miles) is how far you’ve travelled. Watts are like speed; kilowatt-hours are like distance.
- A watt is a rate: one joule of energy per second.
- A kilowatt (kW) is 1,000 watts.
- A kilowatt-hour (kWh) is the energy used by a 1 kW appliance running for one hour: 3.6 million joules.
Converting between the two
kWh = watts × hours ÷ 1,000
| Appliance | Power | Time | Energy |
|---|---|---|---|
| Space heater | 1,500 W | 2 hours | 3 kWh |
| Kettle | 3,000 W | 3 minutes (0.05 h) | 0.15 kWh |
| LED bulb | 10 W | 100 hours | 1 kWh |
| Laptop | 50 W | 8 hours | 0.4 kWh |
Going the other way, a device that uses 76 kWh over a year (8,760 hours) has an average power of 76,000 ÷ 8,760 ≈ 8.7 W. That is the median internet router/box measured by ADEME, which explains why such a small device still shows up on the bill: see how much electricity a router uses.
Why it matters for your bill
You pay for energy, in kWh. At the US average residential price of 18.31¢/kWh (EIA, July 2026):
- The 3,000 W kettle boiling for 3 minutes costs about 3¢.
- A 10 W device left on around the clock uses 87.6 kWh a year: about $16.
High wattage means high cost per hour, but what really counts is wattage × time.
Other units you may see
- Amps (A): electric current. Watts = volts × amps for simple resistive loads. Use our watts to amps calculator.
- Volt-amperes (VA, kVA): apparent power, used for sizing supplies and UPS units; equal to watts when the power factor is 1.
- Megawatt-hours (MWh): 1,000 kWh, used for large-scale consumption and wholesale prices.
- Therms (US gas bills): about 29.3 kWh of heat per therm.
Putting it into practice
Find the wattage on the rating plate, estimate how many hours a day the appliance runs, and multiply. Or skip the arithmetic with our appliance running cost calculator.