Garages are the one room where people routinely undersize a heater and then decide heaters do not work. The reasons are structural: the ceiling is usually 9 to 10 feet or higher, the walls and door are often uninsulated, the slab is a huge cold mass, and the big door leaks air at every seam. The results below assume a 9-foot ceiling, poor insulation, and a cold climate, because that is what most garages are. If yours is insulated and drywalled, switch the insulation to 'average' and the load drops by about 17%.
The typical two-car garage is 20 by 24 feet (480 sq ft) or 24 by 24 (576 sq ft). In a cold climate with poor insulation that is roughly 35,000 to 43,000 BTU/hr, which is 10 to 12.5 kW of electric heat or a small gas unit heater. That is far beyond a plug-in heater. A 240 V garage heater at 5,000 W (17,000 BTU) will take the chill off a well-insulated single garage but will not hold a big uninsulated two-car garage anywhere near 65°F in January.
The decision between electric and gas comes down to how you use the space. If you want the garage warm all winter for a workshop, a vented natural gas or propane unit heater costs a fraction to run per BTU. If you want to knock it from 30°F to 55°F for a couple of hours on Saturday, a 240 V electric heater wins on price and simplicity, and slight undersizing is tolerable because you are heating people, not holding a set point.
Heating BTU by garage size
Baseline: average sun, poor insulation, cold climate, 9 ft ceiling.
| Floor area | Best case | Baseline | Worst case | Unit to buy |
|---|---|---|---|---|
| 240 sq ft | 12,000 | 17,800 | 18,700 | 20,000 BTU≈ 5,861 W |
| 400 sq ft | 20,000 | 29,700 | 31,200 | 30,000 BTU≈ 8,792 W |
| 480 sq ft | 24,000 | 35,600 | 37,400 | 40,000 BTU≈ 11,723 W |
| 576 sq ft | 28,800 | 42,800 | 44,900 | 50,000 BTU≈ 14,654 W |
| 660 sq ft | 33,000 | 49,000 | 51,500 | 50,000 BTU≈ 14,654 W |
| 800 sq ft | 40,000 | 59,400 | 62,400 | 60,000 BTU≈ 17,584 W |
Best case = good insulation and sunny exposure; worst case = poor insulation and heavy shade.
Garage-specific advice
Insulate the garage door first
An uninsulated steel door is about R-1; an insulated one is R-12 to R-18. Adding a foam kit to the door and weatherstripping the perimeter can cut the heating load 20–30% for under $200.
Match BTU to the electrical you have
5,000 W (17,000 BTU) needs a 30 A 240 V circuit; 7,500 W (25,600 BTU) needs 40 A; 10,000 W needs 50 A. If you cannot add a circuit that size, gas is your route to the BTU you need.
Use infrared for spot work, forced air for the whole space
Radiant tube or infrared panel heaters warm you and the floor directly, which lets you run a smaller unit if you work in one spot. Forced-air heaters warm the air and need the full calculated BTU.
Common questions
How many BTU do I need to heat a 2-car garage?
About 24,000 BTU/hr for a 480 sq ft insulated garage in a moderate climate, rising to around 36,000 BTU/hr with poor insulation in a cold climate. That is 7 to 10.5 kW of electric heat.
Will a 1,500 W space heater heat a garage?
No. 1,500 W is about 5,100 BTU/hr, enough for roughly 100–150 sq ft of insulated, conditioned room. It will barely register in a 400+ sq ft garage with a cold slab.
Is a 5,000 W garage heater enough?
5,000 W is about 17,000 BTU/hr. It handles a one-car garage (240–300 sq ft) in most climates and an insulated two-car garage in a hot or moderate climate. For an uninsulated two-car garage in a cold climate you need roughly double that.