Heating a Garage Woodshop: Sizing, Wiring, and Running Costs Compared

We've all been there. It's December, the shop is hovering somewhere in the 40s, and you're wearing gloves you can't actually feel the wood through. At some point, every garage woodworker looks up from the bench and thinks: I need heat, and I have no idea which kind.

The good news is that the answer isn't mysterious. It's arithmetic plus wiring, and both are manageable if you do them in the right order. Here's the sequence I'd follow, with the real numbers for the four heater types most of us actually end up buying.

Start With Sizing, Not Shopping

Before you compare units, figure out how many watts your shop actually needs. The common rule of thumb, as EcoFlow's cost guide lays out, is about 10 watts per square foot for a well-insulated garage and 15 to 20 watts per square foot for minimal or no insulation. Ceilings over 10 feet? Add 25%.

That insulation number is the one that stings. A poorly insulated garage can need twice the wattage of an insulated one — the difference between a 3,000W heater and a 6,000W heater for the exact same space. So before you spend $1,500 on a heater, spend an afternoon on weatherstripping and insulation. EcoFlow notes that alone can cut your heating bill by as much as half. Best money you'll never spend on a heater.

For scale: a one-car garage (200 to 250 square feet) needs roughly 2,000 to 3,750W. A two-car garage (400 to 500 square feet) needs 4,000 to 7,500W, which requires 240V. A three-car workshop needs 6,000 to 12,000W on a dedicated circuit. King Electric, the Seattle heater manufacturer, also recommends targeting a comfort temperature of 70°F in its sizing calculator and generally recommends 240V on a new dedicated circuit for anything serious.

Why a 5,600W Heater May Not Be Wireable in Your Shop

This is the step most people skip, and it's the one that decides everything.

Fixed heaters are treated as continuous loads, meaning the wire and circuit protection are generally sized for at least 125% of the heater's load — the heater shouldn't exceed 80% of the branch-circuit rating. King Electric's circuit-sizing table shows what that means in practice: a 5,000W/240V heater draws 20.8 amps and needs #10/2 copper on a 30A two-pole breaker. A 5,700W unit draws 23.8A — still #10/2 on a 30A, but right at the limit. A 7,500W heater draws 31.3A and jumps to #8/2 wire and a 40A two-pole breaker.

So a popular 5,600W unit like the Dr. Infrared DR-988 needs an existing 30A/240V receptacle. If your panel has no spare two-pole spaces, or the run to the garage means new #8 or #6 wire, that's the moment a heater becomes "not wireable in your shop." King Electric recommends a licensed electrician for new 240V circuits, and I'd agree — this is the one part of shop improvement I never wing.

One more wiring fact worth taping inside your cabinet: a 240V heater run on 120V produces only about 25% of its rated wattage. A 120V heater on 240V produces roughly four times its rating, which is a genuine fire hazard.

The Running-Cost Math

Here's the formula, from EcoFlow at $0.13/kWh: watts ÷ 1,000 × hours × your rate.

A 1,500W heater running 4 hours a day costs about $0.78 a day, roughly $23 a month. A 5,000W unit the same hours runs about $2.60 a day, call it $78 a month. That's the gap between a small heater and a big one.

And a myth worth retiring: neither 120V nor 240V is automatically more efficient. A watt is a watt regardless of voltage. A 240V unit's real advantage in a big shop is faster recovery — it gets you to temperature sooner, which cuts runtime, which cuts cost.

Electric Forced Air: The Default, For Good Reason

Marc Spagnuolo (The Wood Whisperer) runs Fahrenheat FUH54 and FUH724 fan-forced electric units in his insulated ~950-square-foot, four-car Colorado shop, and reports reaching the mid-60s within 45 minutes on the coldest days, after which the units cycle. On most days he only runs the smaller one. A commenter with a 24×24×8 shop says his electric unit "does a phenomenal job" but "wreaks a little havoc on the electric bill" — which matches the math above.

One operational detail that will save you a mistake: don't kill these heaters at the breaker when you leave. The fan is designed to run until the heating elements cool down.

Gas Forced Air: Low Sticker Price, High Installed Cost

The Modine Hot Dawg HD45AS0111 is the reference here: 45,000 BTU/hr input, 36,900 BTU/hr output, 80% thermal efficiency, 720 CFM, 57 dBA, rated for a 2 to 2½-car garage. Sticker is $1,379 at Total Home Supply — but the flush-mount bracket ($209), thermostat ($45–$55), and required 3-inch vent pipe (not supplied) are extra, and Modine strongly recommends a licensed HVAC technician for installation.

Real installed cost runs much higher. One Wood Whisperer commenter paid around $3,000 installed because the installer had to upsize a gas line to 3/4" and run long stainless vent pipe. Spagnuolo's own first gas bid was over $5,000 thanks to a long line run across terrain and concrete.

For a woodshop specifically, gas forced air needs exhaust and, ideally, sealed combustion with a clean air intake, because of wood dust and finishing fumes. One owner's workflow I like: keep the shop at 50–55°F, bump to 75°F for finishing, shut the furnace off while applying finish, ventilate 30 minutes with the doors open, then reheat.

Infrared and Mini-Splits: Know the Trade-offs

Infrared tube heat warms objects and bodies rather than the air — you can stand in front of one right after startup and feel it on your skin. It's quiet, it doesn't blow dust around, and it has an enclosed combustion chamber. The trade-offs: the tubes are large and must cover a significant area, and warm-up can mean hot and cold spots — a hot face and cold feet. Owners seem to really love them anyway, though one commenter who swapped a tube heater for a gas wall furnace was very pleased with the decision.

Mini-splits are the tempting one: quiet, efficient, and they condition the space when it's in use, which Carrier frames as the efficiency case for intermittent shop time. Carrier sizes them at 12,000 BTU for a one-car garage and 18,000 BTU or more for a two-car garage or high ceilings, and notes garages are typically less insulated than living space. The catch is cold: Spagnuolo, who ran mini-splits in Arizona for five or six years, calls them efficient and quiet but says they "generally won't work very well once the temperature drops to a certain point." His Arizona-bought unit just didn't work out in Colorado.

From My Workshop to Yours

Do this in order: insulate and weatherstrip first, size your wattage honestly, then check your panel before you fall in love with a unit. Electric forced air is the simplest path if you can get a 240V circuit; gas forces you to budget for installation, not just the sticker; infrared and mini-splits both suit specific situations. And before you buy anything big, run the cost formula with your own utility rate — that's the number that decides whether you heat the shop all winter or just the hour before you cut.