What size generator do I need to run a gas furnace?

A gas furnace burns gas for heat, but it still needs electricity. Without power, the blower can't push warm air and the controls can't light the burner. The good news: it needs far less power than most people expect.

How many watts does a gas furnace use?

The big user is the blower fan, which is a motor. The igniter, the draft fan and the control board add a little. Generator makers list furnace fans by horsepower:

Blower size	Running watts	Starting watts	Source
1/8 HP	500	1,000	Honda
1/6 HP	750	1,500	Honda
1/4 HP	900	1,800	Honda
1/3 HP	1,000	1,800	Honda
1/3 HP	700	about 2,100	LCEC worksheet (700 + 1,400 extra to start)
1/2 HP	300–800	800–1,600	Champion
1/2 HP	1,200	2,500	Honda

These charts don't agree, and that's normal. Honda's chart is older. Newer furnaces often have variable-speed blowers that draw less. Our calculator uses 700 running and 1,800 starting watts for a 1/3 HP blower.

A gas furnace is not an electric furnace. Electric furnaces and heat pump backup heat use thousands of watts and need a much bigger, 240 V setup.

A worked example: furnace, fridge, lights and Wi-Fi

• Running: 700 (furnace fan) + 400 (fridge) + 100 (lights) + 15 (Wi-Fi) = 1,215 W
• Biggest surge: the furnace fan, 1,800 − 700 = 1,100 W
• Starting: 1,215 + 1,100 = 2,315 W
• With 20% headroom: about 1,500 running and 2,800 starting watts

A 3,000 W class inverter generator covers this. The calculator above is set up with the same list. For the furnace alone, the numbers are 850 running and 2,200 starting with headroom.

Why an inverter generator is the safer pick

Modern furnaces run on an electronic control board. It lights the burner and then checks for a flame with a flame sensor. That makes them fussier about power than a lamp or a space heater.

Clean power. Generators list "total harmonic distortion" (THD), which measures how rough the power is. Lower is cleaner. Westinghouse's manuals make the gap clear: an open-frame model lists THD of 23% or less, while its inverter model lists 3% or less. Inverter power is closer to what the utility sends, so the furnace's electronics see what they expect.

A solid ground. Some inverter generators have a "floating" neutral, meaning the neutral isn't tied to the frame. Westinghouse's inverter manual warns that devices needing a grounded receptacle pin connection "may not function properly." When a generator feeds the house through a transfer switch or interlock, the home's own neutral-ground bond usually takes care of this. Your electrician will know which setup your switch and generator need.

If your furnace won't light on a generator but works on utility power, power quality or grounding is the usual suspect. Ask an HVAC tech or electrician. Don't guess.

How to connect a furnace to a generator

Most furnaces are hard-wired, with no plug. You have two safe choices:

1. A transfer switch or interlock at your main panel, installed by a licensed electrician. You can then run the furnace, fridge and other chosen circuits.
2. A single-circuit furnace transfer switch, also installed by an electrician. It lets you switch just the furnace between utility and generator power, with an inlet for the generator cord.

Never backfeed the house through a wall outlet or dryer outlet. It can electrocute a utility worker and start a fire. See interlock vs transfer switch.

Check your own furnace

1. Turn the furnace off at its service switch before opening any panel.
2. Look at the blower motor's label, behind the lower access door. It lists HP and amps.
3. Multiply the amps by 120 for running watts. A blower rated 6 A uses about 720 W.
4. Or read the furnace's rating plate, often inside the door. It lists the whole unit's amps.
5. For starting watts, plan on about 2–2.5 times the running watts if they aren't listed.

Will a 2,000 watt generator run a gas furnace?

Often, yes, if the furnace is the main load. The furnace alone needs about 1,800 W to start in our calculator, which fits under 2,000 W. Add a fridge and you'll need more, or you'll have to start them one at a time with little else on. For a bigger margin, see what a 3,500 watt generator can run.

Running it through a cold night

A furnace cycles, so your generator runs at a light load most of the time. That saves fuel. Propane stores well for winter, which is one reason to consider a dual-fuel unit. Our propane tank run time guide shows how to estimate a night's fuel.

In winter it's tempting to run the generator close to the house, or in the garage, to keep it out of the snow. Don't. Put a CO alarm on every level of your home, too.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.