Best Generators for Running a Furnace During an Outage

What a furnace needs from a generator

A gas or oil furnace does not need a huge generator. It needs the right kind of power and the right connection. Three things matter most:

1. Clean power. Furnace control boards are electronic. Many technicians recommend inverter generators or conventional generators with low total harmonic distortion (THD) to avoid nuisance problems and protect electronics.
2. Enough capacity for the blower motor, including startup, plus whatever else you run at the same time.
3. A safe, code-compliant connection. Furnaces are hardwired. You need a transfer switch, interlock, or dedicated furnace transfer switch installed by a licensed electrician.

For the full background, read our winter outage heating guide.

How to size for your furnace

Find the nameplate inside the furnace cabinet door. Look for the blower motor's full-load amps and the furnace's total amperage. Multiply amps by 120 volts to estimate running watts. Motors draw more briefly at startup, so leave headroom.

Then add everything else you want to power at the same time: refrigerator, freezer, sump pump, lights, internet, and any medical equipment. Aim to keep the total comfortably below the generator's rated running watts.

What makes a generator good for furnaces

• Inverter technology or low rated THD. Inverter generators electronically condition their output. Many manufacturers list THD in specifications. Lower is cleaner.
• Dual fuel. Gasoline and propane flexibility is valuable in winter. Propane stores indefinitely.
• Transfer-switch-ready outlet. A 30-amp locking outlet (often labeled L14-30 for 120/240V) connects to many inlets and transfer switches.
• CO shutoff. Look for UL 2201 or PGMA G300 certification.
• Electric start. Pulling a recoil cord in the cold is hard. Electric or remote start helps.
• Cold-weather reliability. Check whether the manufacturer offers cold-weather kits or guidance.

Our picks by type

Connecting to the furnace

Most furnaces are hardwired to a circuit with a service switch nearby. To run the furnace on generator power, one of these must be installed:

• Panel interlock kit plus inlet box: mechanically prevents the main breaker and generator breaker from being on at the same time. See our guide on how to connect a generator to your house.
• Manual transfer switch: a subpanel with switches for selected circuits.
• Dedicated furnace transfer switch: for the furnace circuit only.

Troubleshooting furnace problems on generator power

Furnace starts, then locks out. A common cause is flame sensing trouble related to the generator's neutral-ground configuration, especially with some floating-neutral generators. A properly installed transfer switch or interlock usually resolves it. Have a technician check the connection.

Blower hums but will not start. The generator may not have enough surge capacity, or it is overloaded. Turn off other loads and try again.

Control board errors. Power quality may be the issue. Inverter power usually resolves it.

Fuel planning for winter

Furnaces cycle on and off, so generator load varies. In very cold weather, the furnace may run most of the time. Plan for several days of fuel. See how long a generator can run for refueling guidance.

Inverter vs conventional: the trade-off

Inverter generators produce clean, stable power by converting the engine's raw output to DC and then back to AC electronically. They are quieter and more fuel-efficient at light loads because many can throttle the engine down when demand is low, which matters when a furnace cycles on and off all night. The trade-off is cost: inverters deliver fewer watts per dollar.

Conventional (open-frame) generators run the engine at a fixed speed to produce AC directly. They are louder and less fuel-efficient at partial loads, but they deliver more total watts for the money. Some manufacturers specify low THD on their conventional models, which makes them reasonable for furnaces.

For a house where the furnace is the priority and the rest of the load is modest, a mid-size inverter is the most comfortable choice. For a house that also needs a well pump or large 240V loads, a larger conventional generator with low rated THD can be the practical answer.

Questions to ask your electrician

1. Is my furnace on its own dedicated circuit?
2. Would an interlock kit or a manual transfer switch suit my panel better?
3. What inlet and cord rating matches my generator's largest outlet?
4. Will the connection preserve the home's neutral-ground bond so the furnace's flame sensor works properly?
5. Do I need a permit and inspection where I live?

Maintenance for winter readiness

Run the generator monthly under some load. Change oil on the manufacturer's schedule. Use fuel stabilizer in stored gasoline, or run the carburetor dry before storage if the manufacturer recommends it. Keep a spare spark plug, oil, and the tools to change them. A generator that will not start during a snowstorm is worse than no generator at all, because you planned around it.

What about standby generators?

Permanently installed standby generators with automatic transfer switches start themselves when the power fails and run on natural gas or propane. They are the most convenient way to keep a furnace running, but they cost considerably more and require professional installation and permits. Portable generators with a transfer switch are the budget-friendly middle ground for most homes.

A note on extension cords

You may see advice online to cut the furnace's power cord or wire a plug onto a hardwired furnace so it can be plugged into a generator cord. Codes on this vary by jurisdiction, and improvised wiring creates shock and fire risks. The safe, simple path is a transfer switch, interlock, or dedicated furnace transfer switch installed by a licensed electrician.