Heating Only When You Use It Is Usually Far Cheaper
The biggest decision isn't the fuel, it's whether the garage stays warm all week. Keeping an uninsulated 2-car garage at 50°F while it averages 30°F outside costs roughly $315 a month on propane in this calculator. Heating the same garage to 60°F for 8 hours a week costs about $28. A heater that runs only during use pays for warm-up time, but it skips the 160 hours a week nobody is in there.
Keep-warm heating makes sense mainly to protect something: water lines, a drain, paint and chemicals, a battery bank, or a workshop where glue and finishes need to cure. For that, 40°F is usually enough, and it costs about half what 50°F does.
Insulation Matters Most If You Heat All the Time
An uninsulated 2-car garage loses heat about three times as fast as a fully insulated one, mostly through the open ceiling and around the garage door. Insulating walls alone helps less than people expect when the ceiling and door are bare. If you plan to keep the space warm, insulate the ceiling and add an insulated door or a door insulation kit first; if you only heat it a few hours a week, insulation pays back slowly and a bigger heater is the cheaper fix.
Sizing the Heater
The size you need comes from the coldest typical night, not the average. To hold 60°F at 0°F outside, an uninsulated 2-car garage needs about 38,000 BTU an hour — a 45,000 BTU propane or natural gas unit heater. Fully insulated, the same garage needs about 13,000 BTU an hour, which a 4–5 kW, 240-volt electric heater can supply. A standard 1,500 W plug-in space heater puts out only about 5,100 BTU an hour, which is why it barely dents a cold, open garage.
Fuel and Safety
- Propane or natural gas unit heater (vented): the usual choice for large or uninsulated garages. Needs a flue or sidewall vent and, for propane, a tank or cylinder hookup.
- 240-volt electric heater: simple to install and clean, but electric resistance heat costs the most per BTU.
- Ductless mini-split heat pump: the cheapest to run for keep-warm heating in an insulated garage, and it cools in summer, but the most expensive to buy.
- Unvented propane or kerosene heaters put all their combustion moisture and exhaust into the space. They rust tools, can raise carbon monoxide and are not meant as a permanent heat source; use them only briefly with ventilation and a CO alarm.
For ready-made monthly figures by garage size and insulation, see how much it costs to heat a garage.