Garage heater size

A tight, insulated 2-car garage in Chicago needs about 4,700 WCComputed by our engineheatLossDesign(), tight envelope, IL-chicago h99Report a problem to 13,700 WCComputed by our engineheatLossDesign(), leaky envelope, IL-chicago h99Report a problem if it's left bare and uninsulated. The same size garage in Atlanta only needs 2,600 WCComputed by our engineheatLossDesign(), tight envelope, GA-atlanta h99Report a problem to 7,500 WCComputed by our engineheatLossDesign(), leaky envelope, GA-atlanta h99Report a problem. This is a planning bracket built from size, insulation and climate alone — not a load calculation for your actual garage.

Wattage brackets by size and climate

Each row is the same detached garage size, computed twice: once at a tight envelope (R-13 walls, R-30 ceiling, an insulated door kit, tight construction) and once at a leaky one (uninsulated walls, a bare drywall ceiling, a plain steel door, leaky construction). The bracket between them is the planning range — an attached garage sharing a house wall needs somewhat less than either end.

BTU/h and watts are the same load — watts is what a breaker and a heater's nameplate are actually rated in. Every figure above comes from the same physics engine behind the calculator, run at each preset's default dimensions with a 10%CComputed by our engineSIZING_MARGIN — lib/planner/constants.tsReport a problem sizing margin already applied.

Heaters that match the tight end of a bracket

If your garage is insulated and sealed like the tight column, check whether its row is listed below. If it's closer to the leaky column, fix the door and ceiling first: the leaky column needs a much bigger heater, and it loses that extra heat every hour the heater runs. Your garage's own number from the calculator beats either column.

No 1,500 W plug-in heater is on this list. Every row here holds the garage warm all day, and a cord-and-plug portable is for attended use only: unplug it when you leave. It suits spot heat while you work; the portable heater guide covers that.

Above 5 kW (3-car Chicago, 4-car Chicago even when tight), one 5 kW unit won't carry it. Compare a bigger 240 V unit on the 240 V page with gas, propane and heat-pump options on the fuel comparison.

Why the same size garage needs 3x more heater

Take the 2-car Chicago row above. Tight, it grades B on the envelope scale; leaky, the exact same footprint grades F:

Here's where the leaky version's heat actually goes:

How to read this, and when it's wrong for you

• Every row assumes a detached garage. Attached to a heated house, the shared wall gives some of that heat back — your real number is usually lower.
• The design temperature is each station's ASHRAE 99% figure — the outdoor temperature it's colder than only about 1% of winter hours, not the coldest night on record.
• Door count and size vary by garage, even within one preset size — two 9 ft doors lose heat differently than one 16 ft door.
• This bracket doesn't know your actual wall type, ceiling insulation or how drafty your garage really is — it only shows the tight and leaky corners.

For a bracket, that's the point — it tells you the shape of the problem before you measure anything. For a heater purchase, run the garage heater calculator with your actual dimensions, and see every class side by side on the electric heater page. Don't buy by wattage alone off this chart. For heater picks by garage type, see the best garage heater page.