Electricity is measured and purchased in units called kilowatt-hours, which are calculated by multiplying the power used by a device in kilowatts by the number of hours it is in use. The cost of electricity depends on the time of day, with peak hours being most expensive and off-peak hours being least expensive. Examples are provided to demonstrate how to calculate the kilowatt-hours and resulting cost for different electrical devices based on their power usage and duration of use.