Watts to kWh: the formula
kWh = watts × hours ÷ 1,000
Watts tell you how fast a device uses power; kilowatt-hours tell you how much energy it used — and that's what the meter counts and the utility bills. Multiply by the days you use it for a monthly total, and by your price per kWh for the cost.
| Device power | 1 hour | 8 hours a day for a month | Monthly cost at 18.19¢ |
|---|---|---|---|
| 10 W (LED bulb) | 0.01 kWh | 2.4 kWh | $0.44 |
| 60 W | 0.06 kWh | 14.4 kWh | $2.62 |
| 150 W (TV) | 0.15 kWh | 36 kWh | $6.55 |
| 500 W | 0.5 kWh | 120 kWh | $21.83 |
| 1,000 W | 1 kWh | 240 kWh | $43.66 |
| 1,500 W (space heater) | 1.5 kWh | 360 kWh | $65.48 |
At the U.S. average residential price for January–August 2026, 18.19¢ per kWh (EIA). Rates run from about 12.5¢ in North Dakota to over 46¢ in Hawaii, so pick your state in the calculator.
kWh calculator for appliances that cycle
The formula assumes the device draws its rated watts the whole time it's on. That's true for heaters, lights and kettles, but not for anything with a thermostat or compressor:
- Refrigerators and freezers cycle on and off; their yearly kWh is on the yellow EnergyGuide label — divide by 365 for a daily figure.
- Air conditioners and heat pumps run harder on hot or cold days. Use our air conditioner cost calculator for those.
- Space heaters on a thermostat run part of each hour once the room is warm — the space heater cost calculator accounts for that.
For a whole-house number, your bill is the best source; the electric bill calculator turns it into a daily and yearly cost.
How many kWh does a house use?
EIA's monthly survey puts the average U.S. home at about 870 kWh a month over the 12 months to July 2026 — roughly 29 kWh a day. The calculator shows what share of that a device adds. Choosing a water heater? Tankless vs tank compares yearly energy costs. As a rule of thumb, anything that heats or cools — water heaters, dryers, space heaters, AC, EV charging — dominates the total; electronics and lighting are small.
Check the wattage of a device first with a plug-in power meter or its nameplate; if the label shows only amps, the watts to amps calculator converts it. Compare rates across the country with electricity rates by state.
