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How Many Watts Does a Furnace Use? Gas vs Electric, and the Generator You Need
A gas furnace uses about 450 to 1,700 watts of electricity while it runs, depending on the size of its blower motor. Almost all of that goes to the blower fan that pushes warm air through your ducts. An electric furnace is a different animal: its heating elements pull 10,000 watts or more, which is more than most portable generators can supply.
If you heat with gas or propane, the good news is that a modest generator or battery can keep your furnace running in a winter outage. Here are the real numbers and what it takes to hook it up safely.
Gas furnace watts by blower size
These figures come from Goodman furnace spec sheets. Goodman lists a minimum circuit ampacity (MCA) for each model, which equals 1.25 x the blower amps plus the inducer fan amps. That means the furnace's real running current is 80% to 100% of the MCA. All of these furnaces run on 115V.
| Blower motor | Goodman furnace line | MCA | Running watts (about) |
|---|---|---|---|
| 1/3 HP | GMH8 | 4.8 A | 440 to 550 W |
| 1/2 HP | GMH8 | 8.8 A | 810 to 1,010 W |
| 3/4 HP | GMH8 | 14.7 A | 1,350 to 1,690 W |
| 1/3 HP | GMS80 | 8.5 A | 780 to 980 W |
| 1/2 HP | GMS80 | 12.9 A | 1,190 to 1,480 W |
Source: Goodman GMH8 and GMS80 specification sheets. Watts are MCA x 115V x 0.8 to 1.0. Motor power factor makes real watts a little lower than this, so treat these as safe upper numbers for sizing.
Find your own number: look at the rating plate inside the furnace door. Multiply the listed amps by 115 for a close estimate.
How much electricity does a gas furnace use per day?
It depends on how long the blower runs. On a cold day, a furnace might run 8 hours total. For a 1/2 HP furnace drawing about 900 watts:
900 W x 8 hours = about 7.2 kWh per day.
Milder days use less. Very cold days, a leaky house, or a fan set to "on" instead of "auto" use more.
What size generator do you need to run a furnace?
A gas furnace runs on 120V, so the generator only needs enough watts for the blower plus whatever else you run at the same time. Leave headroom for the blower motor's startup surge.
- Furnace only: a generator with 2,000 to 3,000 running watts covers most gas furnaces.
- Furnace plus essentials (fridge, lights, Wi-Fi, a sump pump): plan on about 4,500 to 7,000 running watts.
In-stock portable generators that fit:
- DuroMax XP5500HX: 4,500 running watts on gas, dual fuel, CO alert shutoff. Runs a furnace and your essentials. $629
- DuroMax XP8500EH: 7,000 running watts on gas, dual fuel, 30A L14-30 outlet ready for a transfer switch. Runs a furnace plus more of the house. $1,075.99
For whole-house sizing, see What Size Generator Do You Need to Run a House?
How to connect a generator to your furnace safely
Most furnaces are hardwired, so you cannot just plug them into a generator. Never backfeed power through a wall or dryer outlet. It can electrocute utility workers and start fires.
- Furnace transfer switch: the Reliance TF151W 15A Furnace Transfer Switch ($109.99) installs on the furnace circuit and lets you plug a generator in with a standard grounded extension cord. It is rated for up to 1,875 watts. A 20A version, the Reliance TF201W ($114.99), is also available.
- Whole-panel option: a multi-circuit generator transfer switch lets you run the furnace plus your fridge, lights, and well pump from one generator.
- If the furnace will not light on generator power: some furnace control boards need a proper neutral-to-ground connection to sense the flame. If your furnace locks out on a portable generator, have an electrician check the generator's grounding and the transfer switch wiring.
See our Generator Transfer Switch Guide for the full breakdown.
Can a battery or solar generator run a furnace?
Yes, for a while. Use battery Wh x 0.9 / furnace watts = hours.
- The Nature's Generator Lithium 1800 Solar Generator and Transfer Switch ($1,059.99 with a 100 W panel and 4-circuit transfer switch) has a 1,440 Wh LiFePO4 battery. At 600 watts of blower draw, that is about 2 hours of furnace run time (1,440 x 0.9 / 600). Good for short outages.
- For multi-day outages, a home battery in the 10 to 16 kWh range, or a generator, makes more sense. See battery backup for home options.
Need help sizing? Call 1-888-855-7286. Send us the amps on your furnace rating plate and we will tell you what will run it.
What about an electric furnace?
Electric furnaces and heat strips use 10,000 watts or more, usually on a 240V circuit. A portable generator can often run the blower, but not the heating elements. If you heat electrically, plan on a backup heat source for outages or a large standby system sized by an electrician.
Furnace wattage FAQ
How many watts does a gas furnace use? About 450 to 1,700 watts while running, based on Goodman spec sheet data for 1/3 to 3/4 HP blowers. Most of it is the blower fan.
Can a 2,000 watt generator run a gas furnace? Often yes for 1/3 and 1/2 HP furnaces, with little left over for anything else. Check the amps on your furnace's rating plate first.
Will a 5,000 watt generator run a furnace and a fridge? Yes for most gas furnaces. A 4,500 to 5,000 watt generator covers the furnace, a fridge, and lights together.
Do I need a transfer switch to run my furnace on a generator? Yes, if the furnace is hardwired, which most are. A single-circuit furnace transfer switch is the simplest way to do it safely.
How many amps does a furnace use? Roughly 4 to 15 amps at 115V depending on blower size. The rating plate inside the furnace door lists the exact amps.