Electricity cost calculator
Enter the watts of up to four devices, the hours that each runs in a day and your price per kilowatt hour to see the energy and the cost for a day, a period and a year.
This answer is for the example values in the boxes above. 1,200 W for 3 h and 60 W for 5 h, 16 cents a kWh, 30 days. Change the boxes to see your own answer.
A kilowatt hour is 1,000 watts for an hour. The kilowatt hours are watts times hours over 1,000, and the cost is worked out from them on the numbers you typed and rounded once to the cent.
How it works
For each device the calculator multiplies the watts by the hours a day and by the days, and divides by 1,000 to get kilowatt hours. A kilowatt hour is 1,000 watts for an hour, which is 3.6 million joules. The cost is the kilowatt hours times your price.
The price is in cents: type 16 for $0.16. A part of an hour works, so half an hour is 0.5. The cost is worked out on the numbers that you typed and rounded once, to the cent, so the costs of the devices can differ from the total by a cent.
A worked example
A device of 1,200 watts that runs 3 hours a day and one of 60 watts that runs 5 hours a day use 3.6 and 0.3 kilowatt hours, 3.9 in all. At 16 cents a kilowatt hour that costs $0.62 a day.
In 30 days it is 117 kilowatt hours and $18.72. The first device is 108 kilowatt hours and $17.28 of that, and the second is 9 and $1.44. In a year, 1,423.5 kilowatt hours cost $227.76.
What it leaves out
The calculator multiplies what you type. It does not know how often a device switches on and off, and it adds no taxes or fixed charges that a bill may have.
Sources
- NIST Special Publication 811, Appendix B.8: factors for units listed alphabetically, read on 2026-11-01.
Questions
How do I calculate the cost of running a device?
Multiply the watts by the hours that it runs and divide by 1,000 to get kilowatt hours, then multiply by the price of a kilowatt hour. A device of 1,200 watts that runs 3 hours uses 3.6 kilowatt hours, and at 16 cents that costs 57.6 cents.
What is a kilowatt hour?
The energy of 1,000 watts used for an hour, which is 3.6 million joules. A device of 60 watts uses 1 kilowatt hour in about 16.7 hours.
Where do I find the watts of a device?
Look on its label or in its manual. The calculator takes the number that you type and gives no wattage of its own.
Where do I find the price per kilowatt hour?
Look on your electricity bill. Type it in cents, so 16 for $0.16. The calculator gives no price of its own.
How many kilowatt hours does a device use in a year?
Multiply its kilowatt hours a day by 365. Two devices that use 3.9 kilowatt hours a day together use 1,423.5 in a year, which at 16 cents is $227.76.
Published .