What Size Heater Do I Need for a Garage? BTU and Watts
Part of the project planner: Planning heating, cooling and ventilation.
A 20 × 20 ft garage with insulated walls and ceiling needs about 10,000 BTU (2,919 W) to hold 50°F when it is 20°F outside, so a 3,000 W electric heater. The same garage with no insulation needs 20,800 BTU. The charts cover other sizes and conditions.
BTU by garage size and insulation
An 8 ft ceiling, a 30°F temperature rise (for example 20°F outside and 50°F inside), including a 25% margin:
| Garage | No insulation | Walls and ceiling insulated | Well insulated, including the door |
|---|---|---|---|
| 12 × 20 ft | 14,600 BTU | 6,900 BTU | 3,600 BTU |
| 20 × 20 ft | 20,800 BTU | 10,000 BTU | 5,200 BTU |
| 24 × 24 ft | 27,300 BTU | 13,200 BTU | 6,900 BTU |
| 30 × 30 ft | 38,700 BTU | 18,800 BTU | 9,900 BTU |
How the temperature rise changes it
For a 20 × 20 ft garage with insulated walls and ceiling, at 20°F outside:
| Warm to | BTU | Watts | Next standard electric size | Amps at 240 V |
|---|---|---|---|---|
| 20°F (to 40°F) | 6,640 BTU | 1,946 W | 3,000 W | 12.5 A |
| 30°F (to 50°F) | 9,960 BTU | 2,919 W | 3,000 W | 12.5 A |
| 40°F (to 60°F) | 13,280 BTU | 3,892 W | 4,000 W | 16.7 A |
| 50°F (to 70°F) | 16,600 BTU | 4,865 W | 5,000 W | 20.8 A |
Enter your own garage in the garage heater size calculator.
Electric heater sizes, amps and breakers
240 V heaters are wired on their own circuit. The breaker must be 125% of the heater's current, rounded up to the next standard size, and the wire must be sized to match:
| Heater | BTU per hour | Amps at 240 V | Breaker | Cost per hour at 17¢ |
|---|---|---|---|---|
| 1,500 W | 5,118 BTU | 6.3 A | 15 A | $0.26 |
| 3,000 W | 10,236 BTU | 12.5 A | 20 A | $0.51 |
| 4,000 W | 13,648 BTU | 16.7 A | 25 A | $0.68 |
| 5,000 W | 17,060 BTU | 20.8 A | 30 A | $0.85 |
| 7,500 W | 25,590 BTU | 31.3 A | 40 A | $1.28 |
| 10,000 W | 34,120 BTU | 41.7 A | 60 A | $1.70 |
A long run from the panel also loses voltage: see the voltage drop calculator. Have a licensed electrician install a hardwired heater.
Electric, propane or natural gas?
- Electric (240 V) is the simplest: no venting, no fuel, and easy to control. It costs more to run than gas in most places.
- Propane and natural gas heaters are cheaper to run and put out a lot of heat, but must be vented or rated for use in a garage, and need the right clearances. A heater in a garage where gasoline or paint is stored needs to be one designed for that.
- Infrared (radiant) heaters warm you and the objects in front of them directly, which suits working at a bench, but do not warm the whole garage evenly.
- Fit a carbon monoxide alarm whenever a fuel-burning appliance is in or attached to the garage.
Cut the heat you need
- Seal the gaps around the garage door and the bottom edge. This is cheap and reduces the draughts a lot.
- Insulate the walls and the ceiling, and the door if you can. An insulated door cuts the biggest single loss.
- Close the door as soon as you can, and avoid opening it to warm a cold garage.
- Warm only the part of the garage you use, or the object you work on, with a radiant heater.
Mistakes that cost people money
- Buying a heater by watts alone and ignoring insulation. A bigger heater in a leaky garage is just a bigger bill.
- Plugging a 240 V heater into the wrong circuit, or using an undersized wire.
- Using an unvented fuel-burning heater indoors.
- Expecting a heater to warm a cold garage in minutes. Concrete, tools and vehicles hold the cold for hours.
Frequently asked questions
What size heater do I need for a 2 car garage?
For a 20 × 20 ft garage with insulated walls and ceiling and a 30 degree rise, about 10,000 BTU (3,000 W). For a bare, uninsulated garage the same rise needs 20,800 BTU, roughly double, and a bigger rise needs more.
How many watts to heat a garage?
Roughly 3,000 W for an insulated 2-car garage, 5,000 W for a larger or poorly insulated one, and 7,500 W or more for a big shop with a high ceiling.
What size breaker do I need for a 5,000 W garage heater?
A 5,000 W heater draws 20.8 A at 240 V, and the circuit must be sized at 125% of that for a continuous load, so a 30 A breaker with 10 AWG wire. Have an electrician install it.
How much does it cost to run a garage heater?
A 3,000 W heater costs about 51 cents an hour at 17 cents per kWh if it runs non-stop, and a thermostat cycles it on and off, so less in practice.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.