How to Run Your Gas Furnace on a Generator in a Winter Outage

By BackupEngineer. Math review by a practicing power-electronics engineer (not a licensed PE) is pending. General education, not engineering or electrical advice. See the safety notes at the end.

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Most power outages are short. But a big winter storm can bring down lines across a region, or strain the grid itself, and the power can stay off for days in freezing weather. If you heat with natural gas or propane, it's easy to assume you're covered. The catch is that a gas furnace needs electricity to run. This guide covers what your furnace actually needs, the safe ways to power it, how to size the backup, and how to stay clear of carbon monoxide while you do it. Figures and specs are as of the time of writing (October 2026).

How often long winter outages happen

The U.S. Energy Information Administration publishes reliability numbers for every state (EIA Electric Power Annual, Table 11.3). In 2025 the average U.S. customer was without power for about 263 minutes (4.4 hours) over the whole year, counting major events. Leaving out major events, the figure was about 121 minutes. The gap is the big storms. In 2024, a year with major hurricanes, the national figure with major events was about 611 minutes.

Cold-weather states vary a lot from year to year, because one bad storm can dominate the total:

Source: EIA Table 11.3 (SAIDI, minutes per customer per year, including major event days). These are annual totals for all causes, not only winter.

The grid side. Many winter outages come from ice and wind on local power lines, as in the 2007 example below. Extreme cold can also strain power supply. NERC, which oversees bulk grid reliability in North America, said in its most recent winter assessment (2025–2026) that all areas had adequate resources for normal winter peaks. It also said that prolonged, wide-area cold snaps could cause supply shortfalls in several regions, and that four severe arctic storms have hit much of North America since 2021 (NERC 2025–2026 Winter Reliability Assessment; NERC announcement). NERC usually publishes the next winter's assessment in late fall.

Two real examples:

• Kansas ice storm, December 2007. 1 to 2 inches of ice left about 260,000 people without power, many for one to two weeks (NWS Wichita).
• Winter Storm Uri, Texas, February 2021. Generating units failed in the cold, and grid operators ordered the largest controlled load shed in U.S. history. More than 4.5 million people in Texas lost power, some for as long as four days (FERC–NERC report).

So the calm, honest takeaway is this: you probably won't need backup heat often, but multi-day winter outages do happen, in cold states and warm ones, and it's worth having a plan.

Why a gas furnace stops when the power goes out

The gas makes the heat, but electricity runs everything else: the blower that pushes warm air through the ducts, the draft inducer fan that vents exhaust, the igniter, and the control board. No power, no heat.

The blower is the biggest load. Manufacturer sizing charts give typical numbers:

Charts are a starting point, not your furnace. Many newer furnaces use variable-speed (ECM) blowers that ramp up gently and may draw far less. Check the nameplate inside the blower door (with the furnace switched off), the installation manual, or have an HVAC tech or electrician measure it. Our running vs. starting watts guide explains why motors surge at startup and how to read these charts without double-counting.

Safe ways to power a furnace

Here's the most important point in this guide: a furnace is hardwired, so there is no safe way to "plug it in" without the right equipment. Never backfeed your house by plugging a generator into a wall outlet or dryer outlet with a double-male cord. It can electrocute line workers and neighbors and bypasses your home's protection. CPSC warns against it (CPSC safety alert), and utilities say the same. Evergy, for example, tells customers never to connect a generator directly to home wiring or plug it into a wall outlet, and to have an electrician install a transfer switch (Evergy: safety during power restoration).

The safe options, all installed by a licensed electrician with any permit your city or county requires:

1. Inlet box plus a listed interlock on your main panel. The generator plugs into an outdoor inlet; the interlock makes it physically impossible to have the main breaker and the generator breaker on at the same time. You then choose which circuits to run, including the furnace.
2. A manual transfer switch that moves a set of chosen circuits (furnace, fridge, a few lights) between utility and generator.
3. A single-circuit furnace transfer switch. These exist just for this job. For example, Reliance Controls' TF151W is a listed, one-circuit, 15 A switch with a standard 5-15 power inlet, rated for generators up to 1,875 running watts at the time of writing (Reliance Controls). It can be fed from a portable generator or a portable power station, which makes it a good fit if heat is your main worry.

Our companion guide, inlet box and interlock vs. extension cords, compares these options, typical costs, and cords in detail.

• Single-circuit furnace transfer switch: [AFFILIATE LINK] (paid link)
• Generator inlet box kit: [AFFILIATE LINK] (paid link)

Generator or power station?

A portable generator runs as long as you have fuel and can cover the furnace plus the fridge and some lights. It must run outdoors, it's noisy, and you'll be refueling in the cold.

A portable power station (a big battery with an inverter) can run indoors, with no fumes, and is quiet. Its limits are surge and stored energy. Its inverter has to handle the blower's starting surge, and its battery holds only so many kilowatt-hours.

Here's the energy math, with an assumption you should replace with your own number. Say your blower draws the chart value of 875 W and runs half the time on a cold night (that 50% is an illustration, not a measurement):

875 W × 0.5 × 12 hours = 5,250 Wh, about 5.3 kWh for 12 hours

A modern ECM blower drawing a few hundred watts would need far less. A power station holding 2 kWh would cover that 875 W example for only about 4.5 hours (2,000 Wh ÷ 437.5 W average, before inverter losses). That's why many people pair a power station with solar or a generator, or treat it as a bridge for shorter outages. Our power station vs. generator vs. home battery guide walks through the trade-offs.

• Portable power station sized for a furnace blower: [AFFILIATE LINK] (paid link)

Sizing it: add it up

1. List what you'll run at the same time, with running watts: furnace blower, refrigerator, a few LED lights, the internet router, phone chargers.
2. Find the largest starting surge, which is usually the furnace or fridge.
3. Peak = total running + largest additional surge.

Using the Generac chart values from our running vs. starting watts guide:

fridge 700 + furnace 875 + lights/router/chargers 200 = 1,775 W running 1,775 + 2,300 (furnace start) = 4,075 W peak

Then leave some headroom. The calculator does this for you, including propane or natural-gas derates and altitude, and lets you swap in your own nameplate numbers.

Carbon monoxide: the part that matters most

Generators and winter storms are a dangerous mix if the generator is in the wrong place. After Winter Storm Uri, the Texas Department of State Health Services counted 19 carbon monoxide deaths tied to generators, grills, heaters, vehicles in enclosed spaces and ice-blocked furnace vents (Texas DSHS report). CPSC says that an average of about 100 people in the U.S. die each year from carbon monoxide poisoning from portable generators (CPSC, January 2025). The safety rules are simple:

• Outside only, at least 20 feet from the house, with the exhaust pointed away from windows, doors and vents. Never in a garage, basement, shed, crawlspace or on a porch, even with the door open (CPSC; Ready.gov).
• CO alarms on every level of the home, with battery backup (Ready.gov). The EPA recommends alarms with battery backup that meet UL 2034, tested monthly (EPA: winter weather and indoor air quality).
• Clear your furnace vents. Snow and ice can block the furnace's outside intake and exhaust pipes and cause CO to build up indoors. Check them after a storm (EPA).
• Never heat with a gas stove, oven or grill (EPA).

• Consider a generator with CO shutoff. Two industry standards, ANSI/PGMA G300-2018 and UL 2201, require the generator to shut off when CO builds up around it, and UL 2201 also limits CO emissions. A CPSC staff analysis estimated that generators meeting UL 2201 would avert nearly 100% of the deaths in its data and those meeting PGMA G300 about 87% (CPSC, 2022). A CO shutoff is a backup, not a substitute for good placement and indoor alarms.

• Battery-backed CO alarm, UL 2034: [AFFILIATE LINK] (paid link)

If you can't keep the heat on

• To help keep pipes from freezing, let faucets drip and open the cabinet doors under sinks on outside walls (EPA).
• Keep the fridge and freezer closed. Ready.gov says a fridge keeps food cold for about 4 hours and a full freezer for about 48 hours (Ready.gov).
• If the house is getting too cold, go to a warming center or a friend's place. That's a perfectly good plan.

Before the storm: your utility

• Bookmark your utility's outage map and learn how to report an outage and sign up for outage text alerts. Many utilities offer these. Xcel Energy, for example, has outage alert sign-up (Xcel Energy), and Evergy has an outage map and reporting page (Evergy).
• If someone in your home relies on powered medical equipment, ask your utility about its medical-needs or life-support program. The programs we checked focus on extra notifications rather than faster restoration. Xcel, for example, offers extra outage notifications to customers who tell it about a medical condition (Xcel Energy). Evergy has a Medical Customer Program for homebound customers on electric life-support equipment (Evergy).
• Ready.gov also suggests talking to your medical provider about a power-outage plan for medical devices and refrigerated medicines (Ready.gov).
• Either way, a backup plan for heat and equipment still matters.

Safety

• Never backfeed by plugging a generator into a wall outlet; CPSC calls it extremely dangerous for utility workers and neighbors (CPSC). Use a transfer switch or listed interlock installed by a licensed electrician.
• Carbon monoxide: outdoors only, 20 feet away, exhaust pointed away, battery-backed CO alarms inside.
• Working inside a furnace or an electrical panel is dangerous. Leave it to licensed pros.

Disclaimer: General education only, not a design for your home. Chart values are approximate; your equipment's nameplate and manuals rule. Product mentions are examples as of the time of writing, not endorsements. Some links may be affiliate links; we'll always disclose them, and they never change the math.