Sizing
What size heater for a 20x20 two-car garage workshop
A 20 x 20 ft garage with 8 ft ceilings (400 sq ft, 3,200 cu ft) needs about 13,000 BTU per hour, a 4.0 kW electric heater on a 25 A, 240V circuit, to hold 55F when it is 10F outside with R-13 walls and a bare ceiling. Insulate the ceiling and door and it falls to about 4,500 BTU; leave it bare in a cold snap and it climbs to 28,000.
Heat needed to hold 55F in a 20x20 two-car garage
| Insulation | 30F outside | 10F outside | -10F outside |
|---|---|---|---|
| Bare shop: no insulation, steel door, leaky | 11,000 BTU, 3.5 kW | 19,500 BTU, 6.0 kW | 28,000 BTU, 8.5 kW |
| Typical: R-13 walls, bare ceiling, steel door | 7,500 BTU, 2.5 kW | 13,000 BTU, 4.0 kW | 18,500 BTU, 5.5 kW |
| Upgraded: R-19 walls, R-30 ceiling, insulated door | 3,500 BTU, 1.0 kW | 6,000 BTU, 2.0 kW | 9,000 BTU, 3.0 kW |
| Tight: R-19, R-38, insulated door, sealed | 2,500 BTU, 1.0 kW | 4,500 BTU, 1.5 kW | 6,500 BTU, 2.0 kW |
Each cell is heat lost through the exposed walls, ceiling and overhead door plus the heat needed to warm air that leaks in, for a 8 ft ceiling and one 16 ft door. The kW is the same load converted at 3.412 BTU per watt and rounded up to the next 500 W. U-values and air changes are shop conventions, and slab edge loss is ignored.
The circuit that heater needs
A 4.0 kW heater pulls 16.7 A at 240V. A heater runs for hours, so size the conductor for 125 percent of that: 20.8 A, which means a 25 A breaker and 10 AWG copper. Add it to the other circuits in the shop before you price a subpanel: the workshop amps calculator does the sum. Many garage heaters come with a NEMA 14-50 plug; check the plug against the receptacle you will install, and have a licensed electrician confirm the circuit.
If you would rather burn gas in a 20x20 two-car
A vented gas unit heater at 80 percent efficiency needs about 20,000 BTU per hour of input for the typical case above. That is a size to look for, not a recommendation to buy the biggest: an oversized unit short-cycles. Combustion heaters in a garage with a parked car need real ventilation and a carbon monoxide alarm, and an unvented radiant heater does not belong in a closed bay at all.
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Frequently asked questions
Is a 5,000 W heater enough for a 20x20 two-car garage?
A 5 kW heater delivers about 17,000 BTU per hour. For this garage the typical case at 10F outside needs about 13,000 BTU, so 5 kW is enough unless you insulate the ceiling and door first.
Do I need a 240V heater, or will a 120V space heater do?
A 1,500 W 120V heater makes about 5,100 BTU per hour, which is a fraction of this garage's load. 240V units move more heat per amp and per circuit, which is why a garage shop usually gets a dedicated 240V heater circuit.
This page is researched guidance, not professional electrical, engineering or safety advice. Circuits, loads and clearances depend on your house, your service and your town: have a licensed electrician confirm any wiring, and check local code and each manufacturer's manual before you act.
