If you're in the market for a new electric furnace, you want to be sure that you purchase a system that's the right size for your home. If you install a system that's either too big or too small, it won't work with maximum efficiency, and you will lose money on heating or cooling your home. Here's how to calculate the size you need based on the size of your home.
Use the geographic climate zone of the area you live in to determine your heating square footage range. The heating range is expressed as a minimum or maximum number of Btu (British thermal unit) per square foot. The Zone 1 range is 20-25 Btu, Zone 2 is 30-35, Zone 3 is 35-40, Zone 4 is 40-45 and Zone 5 is 45-50. In the northern states of the U.S., Zone 5, your range would be between 45 and 50 Btu per square foot.
Choose a number from your zone range, based upon how well your home is insulated. For instance, for a home in Zone 5 that's well insulated, choose the lowest range value of 45 Btu per square foot. If your home is not well insulated, or if you're not sure about the state of your insulation, choose a higher number — 49 or 50 Btu.
Multiply the Btu number you've selected by the square footage of your home, to calculate the Btu output of the furnace you need. If you live in a 1,500 square foot home, and your Btu per-square-foot needs are 45, your calculation will be 45 x 1,500 = 67,500 Btu output.
Divide your Btu output by 3,400, to find the kilowatt rating of the furnace that you need. You do this because 1kW can produce 3,400 Btu of heat. In our example, for a 67,500 Btu output, you will need a 20kW furnace.