Calculating the cost of electricity per hour is done by finding the kilowatt-hour. Using 1,000 watts for one hour is a kilowatt-hour. The formula to get kilowatt-hours is to take the wattage of the device, multiply by number of hours used and divide by 1,000.
In order to find kilowatt-hours it is necessary to find the cost of electricity in that area. While the average cost of electricity in the U.S. is 12 cents per kilowatt-hour, the cost of electricity varies greatly by region and usage. The wattage of a device is also needed and can usually be found on the device itself, but the information is not always accurate. Many devices do not use the full wattage listed at all times. It is because of this that a watt-hour meter is used to find the average wattage of a device. When plugged directly into the meter, the devices average wattage usage will display on the screen. Rates can also vary by time of year, time of day and each provider's rates.
There is a difference between watts and watt-hours. The number of watts is how much energy a device needs to run, while watt-hours are how much energy is used over a period of time.