How to use the electricity cost calculator
- Type your electricity rate per kilowatt-hour from your bill. If you only want to see how the tool works, press the example rate button, which fills in 0.17 as an illustration and not as your rate.
- Add a row for each appliance with its power in watts, the hours per day you use it and the days per month you use it.
- Read the cost per month and per year at the top, and the table with kWh and cost for every appliance.
- To include standby power, add a row for the device with its standby watts, 24 hours a day.
Formula
kWh per day of use = watts x hours per day / 1,000
kWh per month = kWh per day of use x days per month
kWh per year = kWh per month x 12
Cost = kWh x rate per kWh
A kilowatt-hour is 1,000 watts used for one hour. A 100 watt device running for 10 hours uses 1 kWh.
Worked example: a television and a space heater at 0.17 per kWh
A 100 watt television used 5 hours a day for 30 days: 100 x 5 / 1,000 = 0.5 kWh a day, so 15 kWh a month and 15 x 0.17 = $2.55.
A 1,500 watt space heater used 4 hours a day for 20 days: 1,500 x 4 / 1,000 = 6 kWh a day, so 120 kWh a month and 120 x 0.17 = $20.40.
Together that is 135 kWh and $22.95 a month, or 1,620 kWh and $275.40 over twelve months of the same use.
The 0.17 rate is only an example. Your own rate may be higher or lower, so use the figure from your bill.
Finding the wattage
Most appliances list their power on a label or in the manual. If only amps and volts are listed, multiply them to get watts. The number on the label is usually the maximum, and many devices such as refrigerators, air conditioners and heaters cycle on and off, so their average draw is lower. A plug-in power meter measures the real consumption of a device over a few days, and it is the most reliable input.
Standby power
Televisions, chargers, game consoles and many other devices continue to draw a small amount of power when they appear to be off. Individually it is tiny, but it runs all day. To include it, add a row with the standby watts from the manual or a power meter, 24 hours a day and the days you leave it plugged in. A 5 watt standby draw over a 30 day month is 3.6 kWh.
Why your bill may not match
Real utility bills often include fixed monthly charges, tiered rates, time-of-use pricing, delivery fees and taxes. This tool multiplies every kWh by the single rate you enter, so use an all-in average rate from a recent bill if you want a close match. For comparing running costs of a heating or cooling unit, see the BTU calculator to estimate size first, and for other everyday costs see the fuel cost calculator.
Assumptions and limits
- You enter the rate. The tool does not know your utility or local prices, and the 0.17 example is only an illustration.
- Each appliance draws its label wattage for every hour you enter. Cycling and variable loads can make the true figure lower.
- The yearly figure repeats the month 12 times, with no seasonal change in use.
- One flat rate applies to every kWh. Tiered, time-of-use and demand charges are not modeled.
- Fixed fees, taxes and solar credits are not included.
Frequently asked questions
How do I convert watts to kilowatt-hours?
Multiply the watts by the hours of use and divide by 1,000. A 60 watt bulb used for 5 hours uses 60 x 5 / 1,000 = 0.3 kWh.
Where do I find my rate per kWh?
Look at your electric bill for the price per kilowatt-hour. Some bills show several charges, so you can add them or divide the total bill by the total kWh for an average.
Does an appliance use power when it is off?
Many do. They draw a small standby amount while plugged in. Add a row with the standby watts for 24 hours a day to see how much it adds up to.
Why is my real bill higher than this estimate?
Bills include appliances you did not enter, fixed charges, taxes and sometimes higher rates at peak times. The tool only totals the appliances and rate you give it.
Is the example rate my rate?
No. It is a sample value to show how the tool works. Replace it with the price on your own bill.