How it works
Utilities bill electricity in kilowatt-hours (kWh). One kWh is 1,000 watts used for one hour, or 100 watts for ten hours. To find the cost of an appliance, turn its watts and hours into kWh, then multiply by your rate.
kWh per day = watts × hours per day × number of appliances ÷ 1,000
cost = kWh × price per kWh · year = day × 365 · month = year ÷ 12
Worked example
A 300 W appliance used 6 hours a day uses 300 × 6 ÷ 1,000 = 1.8 kWh a day. At an example rate of $0.15 per kWh, it costs $0.27 a day. Over a year that is 657 kWh and $98.55; per month, about 54.8 kWh and $8.21.
Cost per hour by wattage
| Power | kWh per hour | Cost per hour at $0.15/kWh |
|---|---|---|
| 10 W | 0.01 | 0.15 cents |
| 100 W | 0.1 | 1.5 cents |
| 500 W | 0.5 | 7.5 cents |
| 1,000 W | 1 | 15 cents |
| 1,500 W | 1.5 | 22.5 cents |
Double the rate and every cost doubles. The table makes the pattern clear: small electronics cost very little per hour, while anything that heats, such as space heaters, kettles, hair dryers and ovens, costs the most.
Small loads add up
Low-wattage devices that run for many hours can cost as much over a year as a big appliance used briefly. All at an example rate of $0.15 per kWh:
| Use | kWh per day | kWh per year | Cost per year |
|---|---|---|---|
| 10 W LED bulb, 5 hours a day | 0.05 | 18.25 | $2.74 |
| 5 W standby load, 24 hours a day | 0.12 | 43.8 | $6.57 |
| 1,500 W hair dryer, 12 minutes a day (0.2 h) | 0.3 | 109.5 | $16.43 |
To enter minutes, divide by 60: 12 minutes is 0.2 hours, 30 minutes is 0.5 hours.
Finding the watts
- Nameplate or label. Most appliances list watts on a sticker or plate near the cord, or underneath.
- Amps and volts. If only amps are listed, multiply amps by volts. A 2 A device on 120 V is 240 W; on 230 V in Australia, 460 W. The label figure is a maximum, so real use is often lower.
- Plug-in energy meter. For appliances that cycle, such as refrigerators, freezers and dehumidifiers, a plug-in meter measures actual kWh over a few days. That is more accurate than watts × hours.
- Energy label. Many appliances carry an EnergyGuide label in the US and Canada, or an Energy Rating label in Australia, that gives estimated yearly kWh. Multiply that by your rate.
Appliances that cycle
Refrigerators, freezers, dehumidifiers, water heaters and anything with a thermostat switch on and off on their own. Their label wattage times 24 hours greatly overstates real use. For these, use the yearly kWh from the energy label or a plug-in meter reading, or enter the hours per day that the appliance actually runs if you know it. Seasonal use matters too: a dehumidifier that runs only in summer should be counted only for those months.
Finding your rate
Your bill may split the price into energy, delivery and other per-kWh charges. Add them up, or divide the total of all per-kWh charges by the kWh used that month. Fixed daily or monthly service charges do not change with use, so leave them out. If you have time-of-use pricing, use the rate for the hours the appliance runs.
Practical tips
- Check standby use. Some devices draw power around the clock even when they look off. A few watts for 24 hours a day add up over a year.
- Count all units: enter 10 for ten identical bulbs or strings of lights.
- For heating, see the space heater cost calculator, which accounts for thermostat cycling.
Electrical safety
Do not overload circuits or extension cords. High-wattage appliances should plug directly into a wall outlet. Use outdoor-rated cords and outlets with ground-fault protection outside. Any new circuit or hard-wired appliance should be installed by a licensed electrician to local code.
Frequently asked questions
How do I calculate kWh from watts?
Multiply watts by hours of use and divide by 1,000. A 300 W appliance running 6 hours uses 1.8 kWh.
How much does it cost to run a 1,000 watt appliance for an hour?
It uses 1 kWh, so the cost equals your rate per kWh. At $0.15 per kWh, that is 15 cents.
Why is my real cost different from the calculation?
Many appliances cycle on and off or run below their rated watts, and bills include charges beyond energy. Use a plug-in energy meter for appliances that cycle.