Adding a bit of heat really does make a huge difference. My 520 sq. ft. garage/workshop is heated with 4,000W using a 12,000BTU window-mount A/C-heater combo. My shop usually maintains a temperature in the upper 40s to lower 50s all winter without heat (due to the large concrete slab floor), but with an insulated ceiling and garage door I can raise the temperature up to 68-70 in several hours. On the very rare days that we have really cold temperatures or high winds, I have an old 1,000W space heater (with a frozen mechanical thermostat) that I can plugin to boost the temperature much faster.
If I am really in a hurry I also have an 4,000-18,000BTU Big Buddy propane heater that I have on hand for extended power outtages in case of generator breakdown (or low fuel). A very nice indoor-rated (includes low-oxygen and tip-over automatic shutoff) portable heater unit for anyone looking for such. Adding it's 18,000 BTUs to the nearly 12,000BTUs from the window unit can heat my shop very quickly in a pinch.
If you have not already, insulating your ceiling and garage door -- and weather-stripping the garage door -- will make your shop much easier to heat and help keep down the heating bill.
