Your utility company probably bases your electricity bill on how many kWh, or kilowatt-hours, you use per month. Your appliances, on the other hand, may be labelled for energy usage in watts. This is the amount of electricity each appliance draws when turned on. Kilowatt-hours are an expression of energy used over time, so to calculate kWh you must multiply the energy used in kilowatts by the time the energy was used for. You can calculate kWh over any time span (a day, a month, a year, and so on) to evaluate the cost of using an appliance, but since electrical companies bill monthly, calculating kWh per month on any given appliance is the logical place to start.
Check the appliance in question for labelling that indicates its power usage in watts or kilowatts; you may find this printed or labelled on the appliance casing or on a tag attached to the electrical plug; in some cases it may even be written on the electrical plug itself.
Divide the appliance's power usage in watts by 1,000 to get its power usage in kilowatts. The simplest way to do this is to move the decimal point three places to the left; add zeroes in front of the first digit, if necessary, to accomplish this. If the energy usage was already expressed in kilowatts instead of watts, you can skip this step.
Multiply the appliance's power usage in kilowatts times the number of hours it's typically used in any given day to estimate the kWh per day, or keep a log of how many hours each appliance is used per day and multiple by this number to get a more precise reading.
Multiply the number you just calculated--kWh per day--by the number of days in the current month to get that appliance's approximate kWh use for this month. If you need an exact reading of kWh per month, add together the number of total hours the appliance was used that month, then multiply by its kilowatt energy rating to get the kWh used per month.
The basic formula for calculating kWh is the appliance's energy rating (in kilowatts) times the number of hours it was used over any given time period.