Utilities bill homes in kilowatt-hours (kWh). A kilowatt-hour is the energy a device uses when it draws a thousand watts for an hour, so the watts and hours give the energy, and energy times your price gives the cost. The starting price, 17.3 cents per kWh, is the latest full-year average for US homes from the US Energy Information Administration. Prices vary a lot by state and plan, so use the rate on your own bill.

How to use this calculator

Find the power on the device's label or nameplate. Use the watts figure if it is printed. If the label gives volts and amps instead, multiply them to get watts.
Enter the hours the device is switched on in a typical day, not the hours you spend at home. A heater that is off while you are out uses no energy.
Leave days per week at the starting value for a device that runs every day, and lower it for one that runs only some days.
Enter how many identical devices you have in the count box, such as the number of LED bulbs in a room.
Replace the starting price with the rate from your own bill. If the bill gives dollars per kWh, multiply by a hundred to get cents.
Compare the monthly totals for your main devices with your bill to see which ones use the most.

Worked examples

Space heater

Power: 1,500 W · Hours used per day: 3 h · Days used per week: 5 days

Cost per month: $16.91
Per year: $202.97
Per day used: $0.78
Energy per month: 97.77 kWh
Energy per year: 1,173 kWh
Energy per day used: 4.5 kWh

Window air conditioner

Power: 900 W · Hours used per day: 8 h

Cost per month: $37.89
Per year: $454.64
Per day used: $1.25
Energy per month: 219 kWh
Energy per year: 2,628 kWh
Energy per day used: 7.2 kWh

LED bulbs

Power: 9 W · Hours used per day: 5 h · How many devices: 10

Cost per month: $2.37
Per year: $28.42
Per day used: $0.08
Energy per month: 13.69 kWh
Energy per year: 164 kWh
Energy per day used: 0.45 kWh

Home computer, part of the week

Power: 150 W · Hours used per day: 6 h · Days used per week: 5 days

Cost per month: $3.38
Per year: $40.59
Per day used: $0.16
Energy per month: 19.55 kWh
Energy per year: 235 kWh
Energy per day used: 0.9 kWh

Monthly cost for one device at the starting price and every day of the week

Power	Hours used per day: 1 h	Hours used per day: 4 h	Hours used per day: 8 h
100 W	$0.53	$2.10	$4.21
500 W	$2.63	$10.52	$21.05
1,000 W	$5.26	$21.05	$42.10
1,500 W	$7.89	$31.57	$63.15

Each cell: Cost per month. Inputs not in the table use the calculator's starting values.

How the calculation works

Multiply the watts by the hours the device runs in a day, then divide by a thousand. The result is the kilowatt-hours the device uses in a day. Multiply that by the number of devices to cover the whole group.

Multiply the daily kilowatt-hours by your price per kWh to get the daily cost. The week, month and year scale that daily figure by the days per week you enter, so the monthly figure is the yearly one spread evenly across the months. Treat each result as an estimate.

Finding the watts and the hours

The power is printed on the label or nameplate, usually on the back or underside of the device or on its power supply. If the label gives kilowatts, multiply by a thousand to get watts.

Devices that cycle on and off, such as a refrigerator, a thermostat-controlled heater or an air conditioner, draw less than their label watts over the hours they are switched on. The calculator assumes full power the whole time, so for these it gives an upper estimate. A plug-in power meter shows what a device really draws.

Large loads and air conditioners

Electric car chargers and heat pumps are often rated in kilowatts. Multiply that by a thousand to get watts, and treat the result as an upper estimate, as with any cycling device.

If you are still picking an air conditioner, the BTU calculator sizes one for the room, then use the label on the unit you buy to get its watts here. For wire on a long run, use the voltage drop calculator and your local electrical code, since energy cost does not show whether the wire is right.

Mistakes to avoid

Using the starting price for your own bill. The US average hides large differences between states and plans, so your rate may be quite different.
Mixing up watts and kilowatt-hours. Watts measure how fast a device draws power, while kilowatt-hours measure the energy used over time, which is what the bill charges for.
Entering the hours you are home instead of the hours the device is on.

How to lower your electricity cost

Start with the devices that your monthly totals show using the most. Run them fewer hours, switch off ones you are not using, and replace old bulbs with LEDs. Then re-enter the new hours in the calculator to see what you save.

Typical watts of common household appliances

If you can't find a label, use a typical wattage for a similar device, for example 750 to 1,100 watts for a microwave oven or 120 watts for a flat-screen TV. Then check it with a plug-in power meter if you can.

Common questions

How do I calculate electricity cost?

Multiply the watts by the hours the device runs each day, divide by a thousand, and multiply by your price per kWh. Scale that daily cost up to the days per week, then to a month or a year.

How do I calculate the cost of my electricity bill?

Multiply the kilowatt-hours for the billing period by the energy rate per kWh shown on the bill. The total you pay is higher, because many bills add fixed charges and taxes, so dividing the total by the kilowatt-hours overstates the rate.

Is a high electricity bill normal?

There is no single normal figure, since the home, the climate and the rate all change the bill. Compare the kilowatt-hours with the same billing period last year: more kilowatt-hours means more energy was used, and a higher price for the same kilowatt-hours means the rate changed.

Where do I find the wattage of a device?

Check the label or nameplate, usually on the back, the underside or the power supply. It lists watts, or volts and amps, and volts times amps can run higher than the real draw. A plug-in power meter shows what the device really uses.

How much is one unit of electricity?

On most bills, one unit of electricity is one kilowatt-hour. The price per unit is the energy rate on your bill. The US average for homes is 17.3 cents per kWh.