For those that want a more accurate measurement and don’t mind doing a bit more math, then you need to calculate the amount of air by volume inside your garage. If we use or original example of 20 by 24 garage and multiply it by the height of the ceilings – let’s say 8 feet – then you get the number of cubic feet of your air. 480 square feet multiplied by 8 feet is 3,840 cubic feet.

For increased accuracy, you then need to multiply the R-value of the insulation into the cubic foot number. R-values on the outside of the home can range anywhere from 13 in warm climates to 27 in cold ones.

If you have really good insulation you can simplify this number to 0.5, average insulation to 1, weak to 1.5, and 5 if you have no insulation. Take this number and multiply it by both the cubic feet and the amount of temperature rise that you want. Roughly the formula comes out to this: (Insulation * Cubic Feet of Garage * Temperature Rise) / 1.6 = # of BTU.

In an area of 3,840 cubic feet, with a 35 degree temperature rise with average insulation (or 1) would come out to this: (1 * 3840 * 35) / 1.6 = 84,000 BTU. To get this down into watts, merely divide the amount of BTUs by 3.41. At 84,000 BTU this comes out to approximately 24,633 watts.