🚗What size heater does this garage need?

For an insulated two-car garage in a moderate climate, 30,000 BTU/h is usually enough to hold a working temperature. Uninsulated, or in zone 5 and colder, 45,000 to 60,000 is realistic — and the difference between those two answers is entirely down to the insulation, not the floor area.

A 5,000 W 240 V unit heater is 17,000 BTU/h, which suits a small insulated single garage and is well short of what an uninsulated double needs. Plug-in 1,500 W heaters are 5,100 BTU/h — useful for warming a workbench, not a room. Gas unit heaters and infrared tube heaters exist for garages precisely because electric resistance runs out of capacity here.

For intermittent use, often yes. Infrared warms objects and people directly rather than air, so it feels effective within a minute or two and does not lose everything the moment the door opens. Forced air heats the whole volume, which is better if the garage is a workshop kept warm all day, and worse if you open the door every twenty minutes.

It is the single highest-return change. An uninsulated steel door has an R-value near zero across a large fraction of the room’s exterior surface, and an insulated replacement or a retrofit kit takes it to R-6 or better. Sealing the perimeter and the bottom weatherstrip matters just as much as the panel itself.

Any combustion heater in a garage needs the manufacturer’s specified combustion air and, for most types, a flue. A car running for even a short time in a sealed heated garage is a serious carbon monoxide risk. Fit a CO alarm rated for the temperature range, and never use an unvented heater in a space where an engine runs.

Most people aim for 50 to 60 °F rather than living-room temperature, which is enough for tools, adhesives, paint and comfortable working in a coat. Holding 68 °F in an uninsulated garage through a cold winter is possible and costs more than heating a similar area of house, because the shell is far worse.