I just installed a
17,000 btu Dayton electric unit heater in my 25x28 garage. Last winter I insulated the attic above, this winter I'm in the process of insulating the walls but I'm only half done. Set at its lowest setting, below "low", it keeps the garage above freezing. Set at "low", it's warm enough to work with a sweater on and no jacket, even wash a car in flip flops. My other option was natural gas, but I'm not comfortable running gas pipe and cutting my roof for the flue. This heater needs a dedicated 30 amp 240v circuit, which was no problem for me.
I absolutely love it! It won't heat up a cold garage as quickly as a gas heater, but it will maintain whatever temperature I want.