If you use your garage as a workshop during the winter you most likely have it heated but if you don't use it as a workshop is it really worthwhile ? Heating your garage has several benefits, obviously if your car is kept warmer the oil is less viscous and the coolant not as cold. This makes starting your car easier (on you and the car) and as an added bonus your vehicle heats up quicker as well. Also, todays homes are often built with bedrooms or bonus rooms above the garage. Though building code dictates insulation and finishing in the garage without a heater in the garage these rooms are almost always very cold when the temperature drops.
If you are considering a garage heater you have several options. The least expensive to install is an electric space heater, unfortunately they are also the most expensive to operate. An electric heater that is set to come on and off based on temperature and thermostat can easily have operational costs of $300 to $400 per month during the coldest months. We do not recommend electric heaters for keeping a garage warm throughout the winter. They are best used very sporadically if you need to be in the garage to do a task etc.
Natural Gas is the most economical with monthly operational costs up to 10 times less than electric depending on the style of heater you choose. The two options you have are radiant tube heaters or Forced air unit heaters.
Radiant Heaters. The effectiveness of a radiant tube heater is heavily reliant on its location in the garage. Typically these are mounted at the front of the garage and radiate heat towards the vehicles and large garage door. Radiant heaters heat objects not the air (Just like the sun) and as the objects and floor heat they also radiate the heat so the garage maintains a comfortable temperature. Operational costs of radiant heaters are less than forced air units as they do not suffer from stratification and when you do open the garage door do not have to work as hard to reheat the garage as the objects and floors are already warm and will have retained heat. The only disadvantage to a radiant heater is the amount of heat they put out close to the source. The heat can be quite intense and uncomfortable if you are working under them in a low ceiling workshop environment but the trade off to that is you can actually use them if you are doing wood finishing etc as they do not blow the dust around like a unit heater.
If you use your garage to park your cars, for storage and the occasional project during the winter a radiant tube heater is the perfect choice.
Forced Air Heaters. Forced air heaters operate exactly like a furnace and blow the warmed air around the garage. They heat a garage evenly and todays units are very quiet. The advantages to a forced air heater is they heat the garage just like your home so the garage really becomes a usable room 365 days a year. There are disadvantages to forced air units as well, the most significant being operational costs. A forced air unit will typically cost 30% more than a radiant unit to operate. Warm are stratifies and rises to the ceiling area and when the garage door is open rushes out. This means that even if you only open the garage door for a few minutes the heater has to entirely reheat the garage when the door closes again. Unlike radiant heaters though the effectiveness of a forced air unit is not nearly as reliant on its location in the garage so if installation flexibility is important to you , they should be considered. Also...If your garage is your "man Cave"...or "woman Cave" then the forced air unit is by far your best choice as it maintains a perfect environment.
Regardless of the type of heater you choose you still have to figure out what temperature to keep the garage. Most people keep it at approximately 10 degrees for the winter. (10 degrees in the garage feels toasty warm when its minus 40 outside ) Depending on how you use your garage you may want to adjust that temperature downwards. According to CMHC for a typical double garage that has been insulated the operating costs to run a gas forced air or radiant heater drops by 5% for every degree you drop the temperature below 10 degrees so dropping your temperature to 5 degrees will save you 25 percent in utilities...and still feel warm. Be sure that when you install your heater that it allows you to set temperature as low as you want as many have a threshold that starts at 10 or 12 degrees.