What size generator do I need for a gas furnace?

Honda's calculator lists furnace fans at 300 to 875 W; add your other loads and keep the total under the generator's rated output.

Can a portable generator run an electric furnace?

Electric furnaces heat with electricity and need an electrician's load calculation; small portables are sized for gas furnace blowers.

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Backup generator for a gas furnace: small load, careful hookup

By the Plain Prepper editors. Updated 2026-10-15.

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Quick answer

A gas or oil furnace needs electricity only for its blower, igniter and controls, so a small generator can run it. Honda's wattage calculator lists furnace fan options from 300 to 875 watts, well within an 1,800 W rated inverter generator. The harder part is the connection: furnaces are usually wired in, so NFPA says a qualified electrician should install a properly rated transfer switch. Run the generator outdoors at least 20 feet from the home, as CPSC advises.

How much power a gas furnace needs

Blower is the main load: Honda's wattage calculator lists "Furnace Fan, gas or fuel oil" at 300, 500, 600, 700 or 875 watts, depending on the fan.
Find yours: the furnace's rating plate lists the blower motor; an electrician or HVAC technician can confirm the draw.
Add what else you need: Honda's calculator lists an ENERGY STAR refrigerator or freezer at 192 watts, so a large 875 W fan plus a refrigerator is 1,067 W.
Electric furnaces are different: they heat with electricity, not just move air, and need an electrician's load calculation; a small portable generator is not the tool for that job.

Two inverter generators sized for a furnace

Spec	Honda EU2200i	Honda EU3200i
Rated output	1,800 W (15 A)	2,600 W (21.7 A)
Maximum output	2,200 W	3,200 W
Receptacles	20 A 125 V duplex	30 A 125 V locking, plus 20 A 125 V duplex
Run time per tank	3.2 h rated, 8.1 h at quarter load	3.3 h rated, 8.6 h at quarter load
Fuel tank	0.95 gal	1.2 gal
Noise	57 dBA rated, 48 dBA quarter load	58 dBA rated, 54 dBA quarter load
Weight	47.4 lb	59.1 lb

With an 875 W fan, the EU2200i has 925 W of rated output to spare and the EU3200i has 1,725 W. Both include Honda's CO-MINDER carbon monoxide detection.

Why inverter power suits furnace controls

Many furnaces have electronic controls and ignition. Honda says its inverter generators deliver stable, clean power suitable for computers and other sensitive equipment, as reliable as the power from household outlets. That makes an inverter model a sensible match for a furnace's electronics. Inverter generators also throttle down at light loads; Honda's quarter-load run times above show the fuel benefit when a furnace blower is the main load.

Connecting the furnace: electrician only

Furnaces are usually hardwired, so there is no plug to put into a generator cord.
Transfer switch: NFPA says that if you must connect a generator to house wiring, a qualified electrician should install a properly rated transfer switch following the National Electrical Code and local codes.
Inlet and cord: the EU3200i's 30 A 125 V locking receptacle can feed an electrician-installed inlet and transfer switch through a matching cord; see our cord for a generator guide.
Never improvise: do not plug a generator into a wall outlet or alter furnace wiring yourself.

Fuel for a cold night

Plan fuel from Honda's quarter-load run times, since a furnace blower plus a refrigerator is a light load for these generators. These are plain arithmetic from Honda's figures, not promises:

EU2200i: 24 hours divided by 8.1 hours per tank is about 3 tanks, or about 2.8 gallons of gasoline a day at quarter load.
EU3200i: 24 hours divided by 8.6 hours is about 2.8 tanks, or about 3.4 gallons a day.
Heavier loads burn faster: at rated load Honda lists only 3.2 and 3.3 hours per tank.
Refuel safely: NFPA says to turn the generator off and let it cool before refueling, and CPSC says to store gasoline outside the home, away from fuel-burning appliances.

Running the generator safely in winter

Outdoors only: CPSC says never use a portable generator indoors, including garages, basements, crawlspaces and sheds, even with ventilation.
Distance: CPSC says to keep it at least 20 feet from the home; NFPA warns against standing downwind of the exhaust.
Weather: CPSC says consumer generators are not weatherproof; wait for rain to pass before running one.
Fuel: CPSC says never store gasoline in the home or near a fuel-burning appliance, such as a gas water heater.
Alarms: check that carbon monoxide and smoke alarm batteries work, CPSC adds; see our carbon monoxide detector guide.

What to look for when you buy

Furnace plus fridge: Honda EU2200i

Best for: Running a gas furnace blower and a refrigerator through an outage

Honda lists 1,800 W rated output, inverter power for sensitive electronics, 8.1 hours at quarter load, 48 dBA at quarter load, CO-MINDER and 47.4 lb.

Watch out: Only a 20 A duplex receptacle; a furnace hookup still needs an electrician-installed transfer switch.

See options on Amazon (opens Amazon in a new tab)

More headroom and a 30 A outlet: Honda EU3200i

Best for: Feeding an electrician-installed transfer switch for the furnace and a few circuits

Honda lists 2,600 W rated output, a 30 A 125 V locking receptacle plus a 20 A duplex, fuel injection, CO-MINDER and 8.6 hours at quarter load.

Watch out: 120 V only and 59.1 lb; check the transfer switch's voltage requirements with the electrician.