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.
Heaters that fit a garage
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Sized from the table above. 240 V heaters need their own circuit, and gas heaters must be vented outside and installed to code. Each button opens an Amazon search, so you can compare brands and current prices there.
240 V heater, 1-car garage
5,000 W
17,061 BTU/hr. Covers a 240 sq ft garage in a moderate climate even uninsulated (13,000 BTU/hr), or insulated in a cold one (14,900). Needs a 30 A circuit.
See options on Amazon(opens in a new tab)240 V heater, 2-car garage
7,500 W
25,591 BTU/hr. Covers an insulated 480 sq ft garage in a moderate climate (21,600 BTU/hr), or a 400 sq ft one in a cold climate (24,800). Needs a 40 A circuit.
See options on Amazon(opens in a new tab)Vented gas heater, cold climates
45,000 BTU
Covers the 35,600 to 42,800 BTU/hr an uninsulated 2-car garage needs in a cold climate, and costs far less to run per BTU. Propane or natural gas.
See options on Amazon(opens in a new tab)Garage door insulation kit
17% less heat
Insulating the door and walls takes a 480 sq ft garage from 35,600 to 29,700 BTU/hr in a cold climate. Start with the door, then weatherstrip the edges.
See options on Amazon(opens in a new tab)
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
Heaters are continuous loads, so the breaker is sized at 125% of the draw, with wire rated for it. At 240 V, 5,000 W (17,000 BTU) draws about 20.8 A and needs a 30 A circuit; 7,500 W (25,600 BTU) draws 31.3 A and needs 40 A; 10,000 W draws 41.7 A and needs 60 A. Have an electrician confirm the breaker and wire size for your setup. 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 21,600 BTU/hr for a 480 sq ft insulated garage in a moderate climate, rising to about 35,600 BTU/hr with poor insulation in a cold climate (both with a 9 ft ceiling). That is roughly 6.3 to 10.4 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 240 sq ft one-car garage in a moderate climate even uninsulated (about 13,000 BTU/hr), or insulated in a cold one (about 14,900), and an insulated 480 sq ft two-car garage in a hot climate (about 16,200). An insulated 480 sq ft garage in a moderate climate needs about 21,600 BTU/hr, which takes a 7,500 W heater. For an uninsulated two-car garage in a cold climate you need roughly double that.