Watts are equal to energy change in joules divided by time in seconds. Watts are a unit of power, which is the rate of energy change in a system, or the rate at which work is done. One watt equals an energy change rate of one joule per second.
To charge for a household's energy usage, electrical utility companies use the unit kilowatt-hour, a unit of energy. Multiplying a unit of power, the kilowatt, by a unit of time, the hour, results in a unit of energy. Kilowatt-hours represent the amount of energy an electrical utility company supplies to a household during the length of a billing cycle.