We wrote about electric garage heaters a year ago this month. Most of that post still holds. A year of installs, two real Ontario winters, and a couple of warranty calls later, a few of our defaults have changed. The plug-in 120V units we used to begrudgingly install when the customer insisted are off our list. Sizing for uninsulated detached garages was being chronically underspecced. Thermostat wiring on the better units has gotten more useful. Here is the updated rundown for 2027.
What we still recommend from 2026
The core picture has not changed:
- 240V hardwired beats 120V plug-in for any garage you actually want to keep warm. Twice the wattage at the same current, no cord to fail, no shared receptacle to nuisance-trip a winter circuit.
- A dedicated circuit, not a shared one. The heater is the largest steady load in the garage. It does not belong on the receptacle circuit, the garage-door-opener circuit, or anything else.
- BTU and watt sizing both matter. The product spec sheet will show one or the other and customers usually pick by the wrong number. The math is the same — 1 kW = 3,412 BTU/hr — but the right metric for sizing against a garage is BTU per square foot per hour at design temperature.
- Mounting clearance is real. Eighteen inches from combustibles for most ceiling-mount units. The garage rafter the customer wants the heater up against is exactly the place not to put it.
What we have updated
Plug-in 120V garage heaters are off the list
We still installed a handful in 2026 when customers were renting, when the panel was at limit, or when "I already bought it on sale, just hook it up" came in over the phone. After a year of those:
- The cord at the wall outlet runs hot. A 1500W heater on a 15A circuit is pulling 12.5 amps continuously. Continuous-load math says the conductor and the receptacle want to be sized at 80% of their rating, which is 12A. Every 1500W garage heater we ever installed was sitting right at the edge of that, with no thermal headroom.
- The receptacle fails before the heater does. Two failures in 2026-27 where the receptacle was discoloured at the plug face and the homeowner had been ignoring it for weeks. Both replaced. One had visible scorching.
- It cannot keep an Ontario garage warm anyway. 1500W is enough to take the chill out of a small, well-insulated attached garage. It is not enough to hold an uninsulated detached garage above freezing in January.
For 2027 we are not installing plug-in 120V resistance heaters in any garage we are pulling a permit on. If a customer brings one, we install a dedicated 240V circuit and recommend they upgrade the unit to match.
Sizing for uninsulated garages was light
Our 2026 default for a typical Hamilton or Burlington attached garage was 5 kW for a single-bay and 7.5 kW for a two-car. That holds for garages built since the early 2000s with insulated walls and an insulated overhead door.
For uninsulated or barely-insulated detached garages — the Muskoka workshop bays, the older Stoney Creek and Dundas single-car detached, the 1970s suburban "garage with bare studs" — we were undersizing. The 7.5 kW unit ran continuously through December and January and the garage barely held 10°C. The actual sizing we now use:
- Insulated attached, single car (250-300 sq ft): 4 to 5 kW. About 50 BTU per sq ft. Holds 15-18°C comfortably.
- Insulated attached, two car (450-550 sq ft): 7.5 kW. About 45 BTU per sq ft.
- Uninsulated or partially insulated detached, single car: 7.5 kW, not 5 kW. About 80 BTU per sq ft to overcome the envelope loss.
- Uninsulated detached, two car: 10 kW or two 5 kW units in opposite corners. The two-unit setup heats more evenly than a single central 10 kW and gives you redundancy if one fails.
- Muskoka detached workshop, four-season use: insulation first, heater second. A 7.5 kW heater in an R-5 garage will lose to a 4 kW heater in an R-20 garage every January.
Thermostat wiring has gotten more useful
Last year we defaulted to a line-voltage thermostat for the simplicity. In 2027 the better mid-tier resistance heaters have caught up on controls:
- Low-voltage 24V thermostat input with a built-in control transformer in the heater. Cleaner separation, better temperature control, and the thermostat does not have to switch the full heater current.
- Wi-Fi compatible thermostats that work with the standard 24V control. We have installed Honeywell T6 Pro and Ecobee SmartBuy variants on garage circuits and the smart-schedule case actually pays. Set to 5°C overnight, ramp to 15°C an hour before the EV unplugs in the morning.
- Two-stage control on the 10 kW dual-element units lets the heater run on half power on milder days. Less cycling, less noise from the contactor.
We are still installing line-voltage thermostats on the simpler 4 to 5 kW units where the cost-to-benefit is not there. For anything 7.5 kW or above, the low-voltage 24V thermostat is now standard.
The sub-panel question
For a detached garage being heated, the question of running a single 240V branch circuit from the house versus running a feeder to a sub-panel comes up every quote. Our default has not changed: if the garage already has anything beyond a light and a couple of receptacles, the sub-panel is the right answer. The heater circuit is then one of three or four branches landed locally, and the trench cost has already been amortized across all of them. For background on the trench, conduit, ground-rod, and four-conductor-feeder details, see our detached garage sub-panel post.
For attached garages where the house panel is on the shared wall, a single dedicated 240V circuit from the main panel is fine and a sub-panel would be overkill.
Brands and what we are seeing in the field
From a year of installs and a couple of warranty calls:
- Stelpro garage and shop heaters. Canadian-made, CSA-listed, lineup expanded for 2027. Our default for most installs. The contactor on the higher-output models is rated for the cycle count and we are not seeing early failures.
- King Electric. Workhorse ceiling-mount units. We had one warranty replacement in 2026 on a fan motor; King handled it cleanly. Quiet, reliable, fair price.
- Marley wall-mount. Still installed in workshops where the wall-mount form factor is preferred. Slightly higher cost. Quieter than ceiling-mount.
- Cadet. We have moved away from Cadet on the higher-output units after two thermostat-failure calls in early 2027. Their lower-wattage product is still fine.
Disconnects and the local switch
For hardwired heaters mounted in the ceiling or on a wall, the OESC requires a means of disconnection at the unit unless the breaker is in the same room and visible. In practice that means a small disconnect switch within sight of the heater on most ceiling-mount installs. We mount a 30A pullout or a snap switch in a 4x4 box on the wall below the unit. It also makes service safer — the next time the heater needs filter cleaning or a contactor swap, the disconnect kills it locally without trekking back to the panel.
The insulation conversation we have every time
Every garage heater quote, we ask about the insulation. An R-5 garage with an uninsulated overhead door loses heat faster than a 7.5 kW heater can replace it on a -20°C night. We have walked away from a few quotes where the right next step was R-20 batt and an insulated overhead door, not a bigger heater. The math is straightforward: insulation is paid back in operating cost in two to four winters, and the heater you install afterwards can be one size smaller.
When to call us
If you are planning a garage heater install for this winter or scoping one for next year, we cover Hamilton, Burlington, Waterdown, Ancaster, Dundas, Stoney Creek, Oakville, and the Muskoka cluster. Send us a photo of the garage interior, a panel photo, and the square footage. We will quote the right unit size and tell you whether a dedicated branch circuit or a sub-panel is the cleaner answer. Request a quote.