Running 100 watts for one hour uses 0.1 kWh and costs $0.02 at the U.S. average of 18.19¢/kWh.
At 18.19¢/kWh — U.S. average, Jan–Aug 2026 (EIA).
| Period | kWh | Cost |
|---|---|---|
| Per hour of use | 0.1 | $0.02 |
| Per day | 0.1 | $0.02 |
| Per month | 3 | $0.55 |
| Per year | 37 | $6.64 |
| Where | ¢/kWh | Month | Year |
|---|---|---|---|
| U.S. | 18.19 | $0.55 | $6.64 |
| North Dakota (cheapest) | 12.46 | $0.38 | $4.55 |
| Hawaii (most expensive) | 46.41 | $1.41 | $16.94 |
| Hours a day | kWh a day | Per day | Per month | Per year |
|---|---|---|---|---|
| 1 h | 0.1 | $0.02 | $0.55 | $6.64 |
| 4 h | 0.4 | $0.07 | $2.21 | $26.56 |
| 8 h | 0.8 | $0.15 | $4.43 | $53.11 |
| 12 h | 1.2 | $0.22 | $6.64 | $79.67 |
| 24 h | 2.4 | $0.44 | $13.28 | $159 |
kWh is what you pay for: power (kW) times hours. 100 W is 0.1 kW, so every hour at full power is 0.1 kWh. The U.S. average residential price for Jan–Aug 2026 is 18.19¢ per kWh (EIA), which already spreads delivery and fixed charges over every kWh.
Appliances with a thermostat or compressor only draw full power part of the time; use the duty-cycle field in the calculator for those.
The full method, material data and tips are on the main electricity cost calculator page.