The commercial unit for electricity is one watt for one hour, and is equal to 3.41 B. T. U. Electricity is usually sold on the basis of 1,000 watt hours (called Kilowatt-hours), which is equivalent to 3,410 B. T. U. A watt, as we have learned, is the product obtained by multiplying a current of 1 ampere by an electro-motive force of 1 volt.

From the above we see that the B. T. U. required per hour for warming, divided by 3,410, will give the Kilowatt-hours necessary for supplying the required amount of heat.