If you have a detached garage that is currently not heated, you will need to decide how warm of a garage you want. The warmer you want it, the more energy it will require.
Regardless of the type of energy source you are going to use, it would be wise to evaluate in advance how many BTU (British Thermal Units) will be required to heat your garage and then decide about the necessary performance and quantity of the heaters.
How big is your garage?
The size of your garage will determine how quickly or slowly the garage will heat up. The larger the size of the garage, the longer it will take to heat and vice versa. For example, if you have a one-car garage that is 250 square feet, it will take less time to heat than a two-car garage that is 500 square feet.
The ceiling height is also a factor. Heat rises, so a high garage will also take longer to heat than a low garage.
Other things to take into consideration
Theoretically, a garage that would not lose heat, would need to be heated only once regardless of its size. So thermal insulation is an important factor that will determine the necessary efficiency of the heating system. When used continuously, the heaters have to compensate for the constantly lost heat. The better the insulation, the less power will be necessary to keep the desired temperature in your garage. But if you are going to open the gate once in a while then a lot of heat will be inevitably lost each time regardless of the quality of the thermal insulation of your garage. The heat losses will depend on the dimensions of the gate and the time you keep it open.
And quite naturally, the temperature outside is another crucial factor that will affect the power consumption of your garage heater. The colder it is outside, the more energy you need to use in order to warm up the garage.
So, how many BTU will you need?
After all the considerations mentioned, it should be clear that it is more of a complex question than a simple one. However, it’s safe to say that the bigger your garage and the less thermal insulation you have, the more BTU will be necessary.
An average one-car garage will need around 12000 BTU. For a 2-car garage, 23000 BTU is recommended. You can also find online calculators that evaluate necessary BTU depending on the square footage of your garage, the ceiling height, and the climate zone. But as you realize some factors are anyway left out, you will have to think about them yourself.