All electrical devices have their power measured in watts. For example, a kettle might be 1,500W. To calculate the cost of the power used by an electrical item, the wattage is multiplied by the number of hours it’s used for each day, and then by the number of days per year. The total is divided by 1,000 to give us the number of kilowatt-hours (kWh). The final cost is worked out by multiplying your price per kWh by the amount of kWh’s you use.