Electrical Costs How much does it cost to run an electric device? In this task, you'll determine how much you use an appliance and calculate the cost of your usage. The electric company uses SI units of watts (W) to measure electrical power. A watt is equal to 1 joule per second (j/s). Although the SI energy unit is a joule, the electric company uses kilowatt-hours (kWh) to measure energy. Estimated time to complete: Total time: 1 day Active time: 25 minutes