How to use
- Enter the appliance’s power in watts or kilowatts, or tap a typical appliance.
- Enter how many hours a day it runs and how many days a year.
- Enter your price per kWh from your bill.
- See the cost per day, month and year, and the energy used.
How electricity cost is calculated
This is the method the US Department of Energy gives:
kWh per day = watts × hours per day ÷ 1,000
kWh per year = kWh per day × days used per year
cost = kWh × price per kWh
A month here is one twelfth of the year. One kWh is the “unit” on your bill: 1,000 watts for one hour.
Example
A 1,500 W kettle used for 1 hour a day, at 0.11 per kWh (the DOE’s own example):
- 1,500 × 1 ÷ 1,000 = 1.5 kWh a day
- 1.5 × 365 = 547.5 kWh a year
- 547.5 × 0.11 = 60.23 a year, about 5.02 a month
Cost per hour at 0.15 per kWh
| Power | Energy per hour | Cost per hour | Cost per month at 8 h a day |
|---|---|---|---|
| 10 W | 0.01 kWh | 0.002 | 0.37 |
| 100 W | 0.1 kWh | 0.015 | 3.65 |
| 500 W | 0.5 kWh | 0.075 | 18.25 |
| 1,000 W | 1 kWh | 0.15 | 36.50 |
| 1,500 W | 1.5 kWh | 0.225 | 54.75 |
| 2,000 W | 2 kWh | 0.30 | 73.00 |
For a different price, scale the costs: at 0.30 per kWh they double.
Why your bill may be different
- Label watts are a maximum. Many devices draw less, and fridges cycle on and off. The DOE suggests counting a fridge as running about a third of the time.
- Standby power adds a little for devices left plugged in.
- Bills add more: fixed daily charges, taxes and tiered or time-of-use rates. In some countries the rate per unit rises as you use more, so this is an estimate, not a bill.
Planning a road trip instead? The fuel cost calculator works out petrol or gas cost.