Heater size for a 20 × 20 ft garage, by insulation
An 8 ft ceiling, holding 50°F when it is 20°F outside, with a 25% margin. Amps are at 240 V for the standard electric size.
| Insulation | BTU per hour | Watts | Electric size | Amps |
|---|---|---|---|---|
| None: bare studs, plain door | 20,790 BTU | 6,093 W | 7,500 W | 31.3 A |
| Walls and ceiling insulated | 9,960 BTU | 2,919 W | 3,000 W | 12.5 A |
| Well insulated, door too | 5,196 BTU | 1,523 W | 3,000 W | 12.5 A |
How cold it gets where you live
The same garage with insulated walls and ceiling, held at 50°F, for different design temperatures outside:
| Coldest outdoor temperature | Heat needed | Electric size |
|---|---|---|
| 0°F | 16,600 BTU | 5,000 W |
| 10°F | 13,280 BTU | 4,000 W |
| 20°F | 9,960 BTU | 3,000 W |
| 30°F | 6,640 BTU | 3,000 W |
How warm you want it
At 20°F outside, insulated. The last column is the cost per hour at 17¢ a kWh if the heater runs non-stop at the heat-loss rate:
| Temperature inside | Heat needed | Electric size | Cost per hour |
|---|---|---|---|
| 40°F | 6,640 BTU | 3,000 W | $0.33 |
| 50°F | 9,960 BTU | 3,000 W | $0.50 |
| 60°F | 13,280 BTU | 4,000 W | $0.66 |
| 70°F | 16,600 BTU | 5,000 W | $0.83 |
A 20 × 20 ft garage: the usual two-car size
A 20 × 20 ft garage is a common two-car size. With insulated walls and ceiling it needs about 9,960 BTU to hold 50°F at 20°F outside, so a 3,000 W electric heater on a 240 V circuit, drawing 12.5 amps.
The garage door is the weak point. Insulating and sealing it moves the garage into the well-insulated row, where it needs only 3,000 W. Without any insulation it needs 7,500 W, and at that size a gas heater is often cheaper to run.
Enter your own garage in the garage heater size calculator, or read what size heater for a garage for breakers, fuel and safety.
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Tools & materials for this project
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Frequently asked questions
What size heater for a 20 × 20 ft garage?
About 9,960 BTU, or 2,919 W, to hold 50°F when it is 20°F outside, with insulated walls and ceiling and an 8 ft ceiling. That is a 3,000 W electric heater. With no insulation it needs 20,790 BTU.
How many amps does a heater for a 20 × 20 garage draw?
A 3,000 W heater draws 12.5 amps at 240 V. The breaker must be rated for 125% of that, and the wire sized to the breaker.
How much does it cost to heat a 20 × 20 ft garage?
About $0.50 an hour at 17¢ a kWh if the heater ran non-stop at the heat-loss rate. In practice the thermostat cycles it, and a garage you heat only while you work costs far less.
Will a 1,500 W space heater heat a 20 × 20 garage?
A 1,500 W heater gives 5,118 BTU an hour. That is not enough even when the garage is well insulated (5,196 BTU). It will take the chill off a small area near you.
Related sizes and guides
Related sizes
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.