Will a generator run a gas furnace?

Updated 2026-10-31 · Sources linked inline

Yes, usually, and it needs less power than people expect. A gas or propane furnace burns fuel for heat. It uses electricity only for the blower fan, the inducer fan, the igniter and the control board. The hard part isn't the watts. It's the wiring: a furnace is hardwired into the house, so you can't just plug it into a generator, and the wrong connection can kill a utility worker.

How this guide was made: this is a research guide built on manufacturer specifications and federal guidance, with sources linked throughout. We haven't bench-tested any of the equipment mentioned, and we don't claim to.

How many watts does a gas furnace use?

Look at the blower motor. Manufacturer wattage charts list "furnace fan" by motor size:

Sources: Honda and Champion. Honda's and Champion's figures differ for a 1/2 hp fan because they describe different motors and margins. Planning on Honda's higher numbers is safe.

Newer furnaces often use an ECM (electronically commutated) blower instead of an older PSC (permanent split capacitor) motor. DOE's furnace-fan analysis found ECM fans use less electricity than PSC fans over a year (DOE), but that is annual energy, not a running-watts figure, so check your own furnace's data plate.

Read your own nameplate. The furnace data plate lists maximum amps at 115 or 120 V. Multiply amps by volts for a maximum, for example 8 A × 115 V ≈ 920 W. The real running draw is usually lower, but the nameplate is a reliable ceiling for sizing. Oil furnaces add a burner motor and igniter, so add those.

What about electric heat?

An electric furnace, heat strips or a heat pump are a different class of load. Resistance heating commonly draws thousands of watts at 240 V, and Champion lists a 3-ton central AC at 3,000 to 3,500 running watts and 5,000 to 6,000 starting watts, as a guide for a compressor-based system. A small portable generator won't carry these. If your heat is electric, look at what size generator to run a house and plan for much larger equipment, or for a different heat source.

How big a generator do you need?

Use the same method as our generator size calculator: total the running watts of everything you'll run together, then add the biggest single start surge (starting minus running).

Example: gas furnace with a 1/3 hp blower, plus a refrigerator, lights and a router.

Minimum starting watts: 1,012 + 1,008 = 2,020. With the 20% headroom our calculator offers as an option (a common practice, not a Honda requirement): about 1,214 running and 2,424 starting watts.

A conservative worksheet method adds the largest listed starting figure to the running total: 1,012 + 1,400 = 2,412 W.

What does that mean for a small inverter generator? Honda's EU2200i is rated 1,800 W with 2,200 W maximum output (Honda). It covers the 1,012 W running load and the 2,020 W minimum start, but not the 2,412–2,424 W figures. It is marginal. A generator with a 3,000 W-class rating gives real margin and room for a sump pump or a second appliance.

Honda's sizing guide adds a practical trick: start high-surge appliances at different times. Its example furnace-plus-refrigerator setup needs only the largest single start, not both at once (Honda). In practice that means letting the fridge compressor settle before the furnace blower kicks on, and keeping the microwave off.

The wiring problem

A furnace is wired to a dedicated circuit, often with a nearby disconnect switch. Options, all requiring a licensed electrician and, in most places, a permit and inspection:

1. Manual transfer switch on the furnace circuit. Generac, for example, sells a 15 A furnace transfer switch (model 6375) for portable generators up to 1,875 W, and a 30 A model (6377) rated for generators up to 7,500 W. Its manual describes a unit that is mechanically interlocked so the utility and the generator can't feed the circuit at once (Generac manual).
2. Panel interlock kit with a generator inlet. Lets one generator feed selected panel circuits. This is licensed-electrician work, and the electrician sets the procedure.
3. A different connection method. Some installations use a plug-and-receptacle arrangement where local code allows. Ask the electrician what your code permits.

What not to do:

• Never plug a generator into a wall outlet to feed house wiring. The Consumer Product Safety Commission calls this "backfeeding," says it can electrocute utility workers and neighbors and bypasses household circuit protection (CPSC).
• Never run an extension cord to a hardwired furnace or cut into the furnace's wiring yourself.
• Never run a generator indoors or in an attached garage.

We aren't giving wiring instructions. They depend on your furnace, panel and local code. Ask an electrician for a quote on a transfer switch or inlet before winter, not during an ice storm.

Safety: the furnace and the generator both make carbon monoxide

A gas furnace is itself a CO source if it is poorly vented, so the CDC advises annual servicing by a qualified technician and battery-powered or battery backup CO alarms (CDC). CPSC says to put CO alarms on every level of the home and outside each sleeping area (CPSC). The generator is the bigger risk during an outage. The CDC says to use generators only outside, more than 20 feet from windows, doors and vents, and never in a garage even with the door open (CDC). See generator carbon monoxide safety for the complete rules. Don't run the generator under an eave or beside a window near the room you are heating; exhaust must be directed away from openings.

Fuel: how long will it run?

Furnaces cycle. Honda lists the EU2200i at 3.2 hours at rated load and 8.1 hours at 1/4 load on a 0.95-gallon tank. A furnace-plus-fridge load is well below rated output, so expect the longer end, but Honda doesn't specify the watts behind "1/4 load," and your actual runtime depends on your load. Plan for a refuel at least every night, follow the generator manual's refueling steps, and store fuel only in approved containers away from living space.

Can a battery power a furnace?

Only if it is set up to feed the furnace circuit properly, and the energy requirement is large. A 700 W blower running 40% of the time averages about 280 W. A 1,024 Wh battery with 85% efficiency would last about 3 hours: 1,024 × 0.85 ÷ 280. Most plug-in stations can't safely feed a hardwired furnace without an inlet or transfer arrangement an electrician installs. Check the unit manual first. See generator vs power station.

Test before you need it

1. Find the furnace data plate and note the amps and motor type.
2. Total loads and pick the generator class.
3. Have an electrician install the transfer switch, interlock or other approved connection.
4. Once it's installed, ask the electrician to walk you through a dry run in autumn: generator outside, furnace running, fridge on, then shut down properly.
5. Read the generator manual again, including the CO warning.

For the bigger picture, use the winter storm power outage checklist and see the refrigerator load in how many watts a refrigerator uses.