I have a heater like the one you posted. Let me give you a little tip. The very first thing I would do if I where you is spend a little money and insulate the heck out of your garage. If you just want to do it cheap throw some insulation between the studs and staple some plastic over it. Then throw some insulation up in the attic space if you have one or between the roof rafters if your roof is exposed. I insulated my garage a few years ago and I just turn the heater on to get the garage warm and once it is warmed up I can turn it off for a pretty long time before needing to kick it on again.