How to use this calculator
Find the power rating on the appliance label or charger brick (in watts), enter how many hours a day it runs and over how many days. The electricity price defaults to 17 ¢/kWh, close to the recent US residential average; replace it with the rate on your bill (total cost divided by kWh used gives an all-in rate including delivery charges).
The calculator also works backwards. Enter a monthly cost and your rate to see how many kWh that buys, or measure an appliance's energy with a plug-in meter and enter the kWh to find its average power. For appliances that cycle on and off — refrigerators, air conditioners — use measured kWh or the EnergyGuide yearly figure rather than nameplate wattage.
The formula
- P
- appliance power
- h
- hours used per day
- d
- number of days
- E
- energy used
- p
- electricity price
- C
- cost
Worked example
A 1,500 W space heater runs 3 hours a day for a 30-day month, and electricity costs 15 ¢/kWh.
- Energy = 1.5 kW × 3 h/day × 30 days = 135 kWh.
- Cost = 135 kWh × $0.15 = $20.25.
The heater adds about $20 a month to the electricity bill.
Typical appliance power
| Appliance | Typical power |
|---|---|
| LED bulb (60 W equivalent) | 8–10 W |
| Laptop while working | 30–65 W |
| Gaming PC under load | 300–600 W |
| Window air conditioner | 500–1,500 W |
| Space heater | 1,500 W |
| Level 2 EV charger | 7,200 W |
Assumptions and limits
- Power is constant while the appliance is on. Thermostatic devices draw full power only part of the time.
- Flat rate per kWh; time-of-use or tiered tariffs need separate calculations per period.
Frequently asked questions
How do I convert watts to kWh?
Multiply watts by hours and divide by 1,000. A 100 W device running 10 hours uses 1 kWh.
What does standby power cost?
A device drawing 5 W continuously uses 3.6 kWh a month — about 60 ¢ at 17 ¢/kWh, or roughly $7 a year.
Where do I find my electricity rate?
Divide the total on your bill by the kWh used to get an effective rate. The US EIA also publishes average rates by state each month.
Sources
- US Energy Information Administration — Electric Power Monthly, average retail price
- US Energy Information Administration — Use of electricity in homes