An electric garage heater works by converting electricity you supply to it into heat. As a form of heating is 100% percent efficient as it converts all the electric power it is given into heat, however the electric power plant that produces electricity has an efficiency of around 30% to 40% depending on the fuel it uses. So in your case what you should be your first concern isn’t the efficiency but the cost of electricity. In most cases natural gas or oil is much cheaper and an economic sensible choice.
However there are cases when it is cheaper to use alternatives. Such a special case is your garage. A space that you need to heat for only when you do some job there and you need warmth. Of course seldom you find yourself there, so installing an expensive system with a low operational cost is a bad investment. On the other hand a cheap appliance is what you need even if it would cost you a bit more to operate it. The extra cost isn’t such high when compared to the hundreds of dollars you need for an alternative heating solution.
So electric garage heaters are the best choice when you are trying to heat your garage space.