How to Calculate the Minimum Size of an Electrical Baseboard Heater

Judge the insulation level of the room to be heated on a scale of 1 to 3 with one being poor, 2 being good and three excellent. Older homes built prior to 1970 generally fall into the poor category. Homes built after 1970 and before 1990 will often rate in the good category. New homes usually have excellent insulation.

Measure the room to be heated, length by width and calculate the square footage. Square footage is simply length times width.

Calculate the base wattage required by multiplying by the insulation factor of 12.5 watts for poorly insulated rooms, 10 watts for good or average insulation, and 7.5 watts for excellent insulation. A 10-foot by 15-foot room with average insulation has a base wattage of 10 feet x 15 feet = 150 square feet. 150 square feet x 10 watts = 1,500 watts.

Adjust the base wattage by ceiling height. Add 25 percent to the base wattage for every two feet in height over eight feet. Using our example from the previous step, this room with a 10 foot ceiling requires 1,500 watts + 25 percent, or (1,500 x .25) + 1500 = 375 + 1,500 = 1,875 watts, the size of the baseboard heater to purchase and install.

Adjust the size of the baseboard heater by climate. Colder climates that have temperatures that stay below 20 degrees Fahrenheit for long periods may need to increase the wattage as much as 15 watts per foot, requiring the next size heater or additional heaters.