The amount of electricity required to power one of our space heaters depends on the space heater’s wattage rating and the hours of usage. To calculate how much electricity a space heater uses, multiply the wattage rating by the usage hours. For example, a space heater rated at 1,500 watts (1.5 kilowatts per hour) would use 12,000 watts (12 kilowatts) of electricity for eight hours of usage. (1,500 x 8 = 12,000 watts).