I used to use a 60 Watt one under the sump of my truck overnight, just to keep the underbonnet temp up and the oil a bit thinner to make it a bit easier to crank over in the mornings. I'm not sure it made much difference to the overall temperature inside the garage, and I think I would have needed more than 4 of them to make it warm(er) in there, but that is in a garage that's about 4500 cubic feet.