Garage Heater Size Calculator

For a 24 x 24 ft garage with a 9 ft ceiling and a 50 F rise (0 F outside to 50 F inside): about 13,000 BTU fully insulated, 31,200 with only the walls insulated and 44,100 with no insulation. A warmer climate or a lower target scales it down in step with the temperature rise.

A fully insulated 2-car garage in a cold climate needs about 3,800 W, so a 4,800 to 5,000 W 240V heater covers it. With only the walls insulated it needs about 9,100 W, which means a 10,000 W heater. An uninsulated one needs about 12,900 W, more than any single residential electric heater we list.

A 1,500 W heater makes 5,118 BTU an hour, the most a standard 120V outlet allows. That heats a fully insulated 1-car garage in a mild climate (3,700 BTU from 22 F to 50 F in our example). In a 2-car or uninsulated garage it is spot heat: it warms you at the bench, not the space.

Electric heat is a continuous load, so the NEC sizes the circuit at 125% of the heater's amps. 4,800 to 5,000 W draws about 20 A: a 30 A double-pole breaker and 10 AWG copper. 7,500 W draws 31.3 A: 40 A and 8 AWG. 10,000 W draws 41.7 A: 60 A and 6 AWG. Plug-in heaters go on the circuit their plug is rated for.

A vent-free propane or natural gas heater releases carbon monoxide, CO2 and water vapor into the garage, and an attached garage shares air with the house. The makers require fresh air (a window or door cracked), and you should keep a CO alarm in the garage and one inside by the door. Gas codes also want the burner 18 inches above the floor, and a vented heater sends the exhaust outside instead.

Per 100,000 BTU of heat at US average prices: electricity at 17.3 cents a kWh costs about $5.07, propane at $2.75 a gallon about $3.01 vent-free or $3.76 vented, and natural gas at $1.50 a therm about $1.50 vent-free or $1.88 vented. Electric heaters cost the least to buy and install if you already have 240V in the garage.

Infrared (radiant) heaters warm people and objects in their line of sight, so they feel warm quickly in a drafty or uninsulated garage and keep working when the door opens. Forced-air heaters warm the air, which suits an insulated garage you keep closed and want evenly warm.

It depends on how much heat the garage loses and how many hours you heat it. A fully insulated 2-car garage held at 50 F for 4 hours a day costs about $38 a month with electric heat at the US average rate in a Chicago-like January. The same garage uninsulated costs about $129. Insulation is usually the cheapest heater upgrade.