Furnace Blower Generator Size Calculator

Data reviewed 2026-07-05 ·how we calculate

Here is the most under-appreciated fact in winter outage planning: a gas or oil furnace doesn’t need generator power for heat — it needs it only for the blower fan, the igniter and the control board. That is roughly 700W running for a common 1/3 HP blower, with a starting surge near 1,400W. A modest generator keeps a whole house heated through an ice storm — the catch is that furnaces are hardwired, so getting power to one safely requires an electrician-installed transfer switch or interlock, never an improvised cord.

Size a generator for this load

Computed on typical values; ranges shown below. 25% headroom applied.

Generator safety — read every time

Never run a generator indoors, in a garage, or near windows, doors or vents.Carbon monoxide kills quickly and without warning — place the generator 20+ feet from the house with the exhaust pointed away, and use battery CO detectors inside.
Never backfeed your house through a wall outlet or dryer outlet. Backfeeding can electrocute utility line workers and destroy equipment when power returns.
Connecting a generator to home circuits requires a transfer switch or interlock installed by a licensed electrician. There is no safe DIY shortcut, and this site does not provide wiring instructions.
Use heavy, outdoor-rated extension cords (12-gauge or thicker), fully uncoiled, sized to the load and the distance.

How to size it step by step

Confirm your heat source: these numbers are for gas/oil/propane furnaces where electricity only moves air. An all-electric furnace or heat-pump air handler with strip heat is a 10,000W+ load — a different page of math.
Find the blower size on the motor label inside the blower compartment (or the furnace spec sheet): 1/3 HP is the common case at roughly 700W running, 1,400W starting.
Apply 25% headroom: 1,400W × 1.25 = 1,750W — technically within the 2,000W inverter class, though the next class up buys room for the fridge alongside.
Solve the connection problem properly: furnaces are hardwired, so a licensed electrician must install a transfer switch or interlock (some techs can add a compliant plug-and-receptacle to the furnace circuit). Never backfeed a panel through an outlet.

Pro tips

Check your nameplate first — every figure on this page is a planning estimate, and the label on your specific unit beats any chart.
Locate your furnace switch (looks like a light switch, usually on or near the unit) — with a transfer switch installed, that is where you’ll confirm the furnace is isolated before switching sources.
If your thermostat is battery-free and the furnace loses power, some models forget schedules; a couple of AA batteries in the thermostat keeps settings through the switchover.

The data behind this calculator

Furnace electrical load figures by blower size
Figure	Value	Source
1/4 HP blower running / starting	~600W / ~1,000W	Generator sizing charts
1/3 HP blower running / starting	~700W / ~1,400W	Generac/Champion sizing charts
1/2 HP blower running / starting	~875W / ~2,350W	Generac sizing chart
Hot surface igniter (gas models, during light-off)	~300–500W briefly	White-Rodgers/Honeywell igniter specifications
What this does NOT cover	Electric-resistance furnaces and heat pumps draw 10,000W+ — different problem entirely	DOE Energy.gov heating system data

Duty cycle: In a cold snap the blower cycles with every heating call — expect several starts per hour, each repeating the full surge. The igniter adds a brief ~300–500W on gas models.

Furnace blower generator questions, answered

What size generator do I need to run my furnace in winter?

For a gas furnace with the common 1/3 HP blower: about 700W running and 1,400W starting — 1,750W with the 25% headroom, so even a 2,000W inverter class unit can carry heat for a whole house. Most people size up to the 3,500–4,500W class anyway so the refrigerator and some lights ride along. The constraint isn’t watts; it’s that the furnace is hardwired and needs an electrician-installed transfer switch or interlock to connect safely.

Why can’t I just plug my furnace into the generator?

Because it has no plug: furnaces are wired directly to a circuit, and the safe ways to energize that circuit from a generator are a transfer switch or a panel interlock — both electrician-installed. Some jurisdictions also permit a technician converting the furnace to cord-and-plug connection on a dedicated receptacle. What is never acceptable is backfeeding an outlet with a double-male cord, which can kill line workers and is illegal everywhere. Budget the electrician visit as part of the generator’s price.

Does this math work for heat pumps and electric furnaces?

No, and pretending otherwise would be dangerous budgeting. A heat pump’s compressor is a central-AC-class load (see that page), and its backup strip heat or an electric furnace draws 10,000–20,000W of pure resistance — standby-generator territory. If your heat is all-electric, the portable-generator winter plan is usually: run the fridge and lights, heat one room with a kerosene/propane indoor-rated heater, and let the house go cool.

How much fuel does heating a house this way cost per day?

Less than people expect, because the furnace burns gas for the actual heat. A 1/3 HP blower at a 50% winter duty cycle averages ~350W — call it 8–9kWh per day, or very roughly 2 gallons of generator gasoline. The furnace’s own natural-gas supply is almost never interrupted by the same storms that cut power, which is exactly why the blower is the highest-value 700W in the whole winter plan.

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