How running cost is calculated
Your utility charges for energy in kilowatt-hours (kWh): 1,000 watts running for one hour. Multiply the power by the time, then by your price.
Cost per day = kWh per day × price per kWh
Watts = amps × volts
A month here is an average month, 365 ÷ 12 = 30.42 days, and a year is 365 days. If you use something only on some days, the "days used per week" box scales it down.
Worked example
A 1,500 W space heater running 8 hours a day uses 1,500 × 8 ÷ 1,000 = 12 kWh a day. At $0.15 per kWh, that's $1.80 a day, $1.80 × 365 ÷ 12 = $54.75 a month, and $657 a year.
Typical wattages
The appliance list uses typical values from the US Department of Energy. Your appliance's real wattage is on its label or in the manual, and it's better to use that. Many appliances don't draw full power all the time:
- Refrigerators cycle on and off. The DOE suggests dividing the label wattage by three to estimate actual use, which is why the preset uses 8 hours a day.
- Heaters, AC and dehumidifiers cycle with the thermostat, so real hours at full power are often less than the hours they're switched on.
- Electronics on standby still use a few watts around the clock.
Limitations
Your bill may also include fixed monthly charges, taxes, and rates that change with time of day or with how much you use (tiers). This calculator uses one flat price per kWh. For the most accurate number, divide your bill's energy charges by the kWh used. To measure an appliance directly, a plug-in electricity usage monitor shows its real kWh.