Garage Heater Wattage — What Size Do You Need?
Use this guide to find the right heater size for your garage. Avoid underpowered or oversized units.
A 1-car garage (250 sq ft) needs 2,500–6,000 watts (8,500–20,500 BTU/hr).
A 2-car garage (500 sq ft) needs 5,000–12,000 watts (17,000–41,000 BTU/hr).
The low figure is an insulated, finished garage in a mild climate at 10 W/sq ft. The high figure doubles that for an uninsulated garage and adds 20% for a cold climate — which is what most garages actually are.
Garage Heater Size Guide
| Garage Size | Sq Ft | Watts Needed | BTU Needed |
|---|---|---|---|
| 1-Car garage | 250 | 2,500–6,000W | 8,500–20,500 |
| 2-Car garage | 500 | 5,000–12,000W | 17,000–41,000 |
| 3-Car garage | 750 | 7,500–18,000W | 25,500–61,500 |
| Large workshop | 1,000 | 10,000–24,000W | 34,000–82,000 |
Built from 10 W/sq ft — the rule of thumb for an average-insulated finished room. The low figure is an insulated, drywalled garage in a mild climate. The high figure applies our uninsulated multiplier (x2.0) and cold-climate multiplier (x1.2), because most garages are uninsulated and have a large uninsulated door, which the bare 10 W/sq ft rule understates. Watts convert to BTU/hr at 3.412 BTU/hr per watt.
Calculate the Right Heater Size for Your Garage
How to Choose the Right Garage Heater Size
Start at 10 watts per square foot — the rule of thumb for an average-insulated finished room. A 500 sq ft 2-car garage that is insulated and drywalled needs roughly 5,000 watts on that basis. But most garages are not insulated and not finished, and the door alone is a large uninsulated surface, so the bare rule understates them. We apply x1.5 for a partly insulated garage and x2.0 for an uninsulated one, plus x1.2 for a cold climate and x1.4 for a very cold one. That puts a typical uninsulated 500 sq ft garage in a cold climate at about 12,000 watts (41,000 BTU/hr). Use the calculator above for your own numbers.
BTU and wattage measure the same output differently. One watt equals 3.412 BTU/hr. A 5,000W heater delivers roughly 17,000 BTU/hr, which suits a 2-car garage only if that garage is insulated and the climate is mild. See running cost breakdowns for electric garage heaters and propane garage heaters.
Always check your garage electrical panel before buying an electric heater. A standard 120V outlet tops out at about 1,500 watts, so anything above that needs its own circuit, and garage unit heaters of 3,000 watts and up are almost always 240V on a dedicated circuit. A 12,000 watt heater draws 50 amps at 240V and needs a circuit sized for a continuous load — often more capacity than an older panel has spare. Propane and gas heaters avoid that limit entirely for larger garages and workshops. Compare all types with the garage heater cost calculator.