The amount of power that your garage heater needs depends on the size of the space that you intend on heating. Wattage is one of the most common power units out there, but British Thermal Units (BTU) is also widely used. You can use either to determine the size of the heater that you require for your garage. Watts are easy to calculate as you just need to think 10 watts per square foot of space. The calculations are slightly more complicated when it comes to BTU. You can convert the amount of watts into BTUs by multiplying by roughly 3.41. If you want to be even more accurate, you can calculate the amount of air by the volume inside your garage. Even more accuracy can be achieved by multiplying the R-value of the insulation into cubic feet.