I have a 18x18 garage. When I bought the house they put insulation in the walls but left the ceiling bare. I was working out in the garage one day and I could see my breath. I decided that was enough.
My father came down and helped put up drywall. My father in law tossed in an electric baseboard he had. I set the thermostat at 7 or so around late fall. And turn it off once the temperature stays warm enough. Nothing freezes and it is decent in there. If I was going out there to hang out I would have to turn up the heat a bit. If I was building something or working out I would crack a window because I find it too warm.
My wife never complained about an increase in the power bill. That was 10 years ago I think. I didn't bother looking at it. I wanted heat in the garage. It hasn't been bad. Its well worth having some kind of heat in there. Even just for some of the products that might be in there that don't like to be frozen.
Searching for my first Jeep
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2012 JK Sport 2 door 6 speed