Yes...you use the heater output rating (80%) that's the btu's the heater puts into the building (the rest goes up the flue). Terms like well insulated don't count. You need to figure the square footage of the areas and know the R and then U values of the areas. Then the design temps and it's just simple math after that. As an example your garage ceiling with an R30 would need 2539 btu's an hour to keep it 70 inside at 0 outside. The Door at R3 will require 4480 btu's an hour for the same temp differential. It works like this...1 divided by R Value = U value..... U value X sq footage of the area X desired temp rise = btu's required. Your door for example...16x12 = 192 sq ft, so 1 divided by R3 = .3333U's.... 192 x .3333 U's = 63.99 btu's per degree of temp rise. 0 outside...70 inside = 70 degrees rise. So 63.99 btu's X 70 = 4480 btu's an hour for the door.
I have a 24x34x11 garage here in Minnesota and have no problem keeping it 70 with a 45,000 btu input (36,000 btu output) unit heater. That said I have NO desire to be trying to heat my garage to 70 when it's -20 outside. I heat all winter and keep my garage 45 except when I'm working in it.
That's a little bit of simplification, since you need to figure for infiltration too, but you should get the idea.
Last winter I figure it cost me about $250 to heat my garage for the winter. I used to take my car and truck to the car wash twice a week (each) during the winter @ $6 every time, now I hand wash them in the garage at home, so I figure the heating cost is a wash
