- Calculate the square footage of the room you wish to heat by multiplying the room's length by its width.
- Measure the height of the ceiling. For every 2 feet above 8 feet, you will need to add 25 percent to the total wattage. For example, a room with 10-foot ceilings would require 25 percent more wattage, while a room with 12-foot ceilings would need a 50 percent increase.
- Determine the quality of your insulation. An average home will require 10 watts per square foot. An older home or one built to less expensive specifications will require 12.5 watts per square foot. A home with above-average insulation (grade R-19 wall insulation and R-38 ceiling insulation or higher) will require 7.5 watts.
- Multiply the square footage of your room by the wattage per square foot determined by your insulation. For example, a 10-by-10 room with average insulation will require a 1,000 watt heater.
- Adjust the total based on ceiling height. For example, a 10-by-10 room with average insulation and a 10-foot ceiling would require a 25 percent increase, or 1,250 watts.
- If you live in a cold climate, with temperatures falling well below freezing for extended periods during winter, purchase the next largest available heater.
- If your calculations fall between two available heaters, purchase the larger of the two.