In order to calculate how much it costs to run a 1500 watt heater for 24 hours, we need to take a few variables into consideration. A 1500 watt heater operates with a power consumption of .081 kilowatts per hour. This means that the heater is using 8.1 kilowatt-hours per day. Multiply this number by 365 days and you get 3,829 kilowatt-hours of energy used in a year.