Electricity Cost Calculator: kWh and Cost to Run Any Appliance

How to find the numbers to enter

1. Find the wattage. The Department of Energy (DOE) says it "is usually stamped on the bottom or back of the appliance, or on its nameplate." If the label lists amps instead, multiply amps by volts. Most U.S. appliances use 120 volts; larger ones, such as clothes dryers and electric cooktops, use 240 volts, DOE notes.
2. Use the setting you actually run. The Lasko 754200 ceramic heater, for example, lists 1,500 W on high and 900 W on low, with a current of 7.5 to 12.5 A at 120 V (12.5 × 120 = 1,500 W).
3. Count the devices if you run several identical ones on the same schedule.
4. Estimate the hours a day it runs, up to 24. For anything with a thermostat or a compressor, count running time, not plugged-in time (see "Appliances that cycle on and off").
5. Enter the days a month and the months a year. Every day is 30, or 30.42 (365 ÷ 12) to match a full year. Once a week is 4.33 days a month (52 ÷ 12). For a seasonal device, enter only the months you use it.
6. Add your rate for the cost, in dollars per kWh from your bill (divide cents by 100). The calculator has no built-in rate.

kWh a month by wattage and hours of use

One device used every day of a 30-day month. Multiply any cell by your rate for the monthly cost. The wattages are examples; read your own label.

Turned around, a device uses 1 kWh in 1,000 ÷ watts hours: 10 hours at 100 W, 1 hour at 1,000 W, and 40 minutes at 1,500 W.

Watts from amps and volts

DOE notes the amps "might be stamped on the unit in place of the wattage, or listed in the owner's manual or specification sheet."

Where to find your electricity rate

Our calculator never assumes a rate. EIA says "Electricity prices vary by locality," are usually highest in summer, and that "Some utilities offer their customers time-of-day pricing." Look at your own bill:

• The per-kWh charges. Penn State's guide to reading an electricity bill shows separate charges for supply ("for generating the electricity") and delivery ("to maintain the poles and wires"). If both are billed per kWh, add them: that's what one more kWh costs.
• The all-in rate. Divide the bill's total by the kWh used. In Penn State's example, this comes out "much higher than the electric charge shown, because we included all the charges, taxes, and fees," so it errs high for a single device.
• Time-of-day plans. If you pay more at peak hours, use the rate for the hours your device runs.
• No bill handy? EIA's Electric Power Monthly lists average prices by state and type of customer (Table 5.6.A). Your state's residential average is a stand-in, not your rate.

Appliances that cycle on and off

DOE says "The wattage listed is the maximum power drawn by the appliance." A device with a thermostat or a compressor switches on and off, so the hours it is plugged in overstate its use.

• Refrigerators. To estimate the hours a refrigerator runs at its maximum wattage, DOE says to "divide the total time the refrigerator is plugged in by three." A fridge plugged in all day counts as 8 hours.
• Heaters with a thermostat. The Lasko heater's thermostat has 11 settings to maintain a chosen warmth, so 8 hours switched on at 1,500 W is an upper limit unless it heated nonstop. Heating a whole house is a different job: the furnace size calculator estimates the heat a home needs in BTU per hour.
• EnergyGuide labels. The Federal Trade Commission lists them on appliances including refrigerators, freezers, clothes washers, dishwashers, televisions, water heaters, and room air conditioners. Its cost figure is "only an estimate, based on typical use and a national average price for energy." If the label gives kWh per year, divide by 12 and multiply by your own rate.
• ENERGY STAR listings. ENERGY STAR's data on certified refrigerators gives each model's annual kWh "under typical conditions"; real use varies with door openings, room temperature, and how full it is kept.

Measure it with a plug-in electricity monitor

According to DOE, "Electricity usage monitors are easy to use and can measure the electricity usage of any device that runs on 120 volts." Plug the monitor into the outlet and the device into the monitor, and it displays the watts. Some also estimate the cost if you enter your rate. For 240-volt appliances such as clothes dryers, DOE suggests a whole-house energy monitoring system.

A monitor also catches standby power. DOE notes that "Many appliances continue to draw a small amount of stand-by power when they are switched 'off,'" which you can stop by unplugging the device or switching off a power strip.

Tips and common mistakes

• Typing cents as dollars. A rate of 15 reads as 15 dollars per kWh and makes every cost 100 times too high. The calculator reminds you when the rate is 1 or more.
• Using the high setting for a device you run on low. A heater on 900 W uses 40 percent less than on 1,500 W.
• Treating a seasonal device as year-round. Set the months to the ones you use it, and note that 30 days × 12 months is 360 days, not 365.
• Comparing air conditioners of different sizes. Size a window unit to the room first with the AC size calculator, which works from the room's area (the square footage calculator adds up L-shaped rooms), then compare the wattage of units that size here.
• Ignoring the room itself. If a heater or air conditioner runs for hours in a room under the attic, check the insulation up there: the attic insulation calculator shows how much ENERGY STAR recommends adding for your climate zone.