How many watts does a furnace use?
It depends on the fuel. A gas, propane, or oil furnace uses roughly 400 to 800 watts of electricity while it runs, and an electric furnace uses 10,000 to 20,000 watts. The reason for the split is simple: in a fuel-burning furnace the electricity only moves air and lights the burner, while in an electric furnace the electricity is the heat. So the first thing to check is what your furnace burns. If there is a gas line or an oil tank, you have the low-wattage kind; if the only thing feeding it is a fat 240-volt circuit, you have the high-wattage kind.
This is why a furnace can be either the lightest or one of the heaviest things to back up in an outage. A gas furnace's 400 to 800 watts is less than a 1,500-watt space heater and sits comfortably within reach of a mid-size battery or a small inverter generator. An electric furnace's 10 to 20 kW is in the same league as running an electric dryer, an electric range, and a water heater all at once, and it is out of reach of any portable power station.
Wattage ratings vary by model and blower motor, so treat these as planning ranges and read the FLA (full-load amps) on the furnace's rating plate for your exact unit. To see where the furnace sits against the rest of your home's load, run the numbers through the solar panel calculator or the how many watts to run a house breakdown.
How many watts does a gas furnace use?
A gas furnace uses about 400 to 800 watts of electricity while running, and almost all of it is the blower fan that pushes warm air through the ducts. The gas burner supplies the heat for free (you pay for it on the gas bill, not the electric one), so the furnace's electrical draw is really just the cost of moving air and running the controls. A furnace with an older single-speed PSC blower motor sits near the top of that range, roughly 400 to 700 watts, while a modern variable-speed ECM blower can idle as low as 75 to 200 watts on its lowest speed and only climb when it ramps up.
The other electrical parts are small and brief. A hot-surface igniter draws 300 to 500 watts but only for a few seconds at the start of each cycle to light the burner, the draft inducer motor runs at 75 to 150 watts to vent exhaust, and the control board and 24-volt transformer together use just a few watts. None of these changes the headline number much: the blower is the load that matters. Across a full cycle a gas furnace typically averages well under 1 kWh per hour of actual burn time.
The part that catches people sizing a generator is the startup surge. A single-speed blower motor pulls 2 to 3 times its running watts for the first fraction of a second as it spins up, so a furnace that runs at 600 watts can spike to 1,200 to 1,800 watts briefly, and if the igniter happens to be firing at the same moment the peak is higher still. Variable-speed ECM motors soft-start and barely surge at all, which is one reason they are easier to run on a battery.
How many watts does an electric furnace use?
An electric furnace uses about 10,000 to 20,000 watts (10 to 20 kW) while heating, because it makes heat the hard way: by forcing current through resistance heating elements, exactly like a giant space heater built into your ductwork. A small unit for a tight home might be 10 kW, a typical whole-home unit 15 kW, and a large one in a cold climate 20 kW or more. On top of the elements, the same 400 to 800-watt blower runs to move the heated air, but it is a rounding error next to the resistance strips.
That load lives on a 240-volt circuit (often more than one) sized at 40 to 100 amps, among the biggest circuits in the house. A 20 kW furnace drawing full power uses 20 kWh every hour it runs flat out, which is why an all-electric home on an electric furnace can see its daily kWh double or triple through the coldest months. Resistance heat has a coefficient of performance of exactly 1.0, meaning every watt in becomes one watt of heat and no more.
This is the central reason a heat pump beats an electric furnace so decisively on running cost: a heat pump moves heat instead of making it and delivers three to four units of heat per unit of electricity, so it does the same job on roughly a third of the power. If you heat with an electric furnace and your bills spike every winter, replacing it with a heat pump is usually the single biggest cut you can make to a winter electric bill.
Can a gas furnace run without electricity?
No. A gas furnace will not run in a power outage even though the heat itself comes from gas, because nearly every other part of it needs electricity. The blower motor needs power to move warm air, the hot-surface or electronic igniter needs power to light the burner, the control board needs power to sequence the cycle, and the modern gas valve is held open electrically and closes the instant power is lost as a safety feature. Cut the power and the furnace goes dark and cold.
The practical takeaway is that a gas furnace in winter is a small but non-optional electrical load during an outage, and it is well worth backing up. Because the running draw is only 400 to 800 watts, a mid-size battery, a solar generator, or a small inverter generator can keep it running, which is exactly the kind of backup that keeps a house from freezing when the grid is down. A few very old furnaces with a standing pilot and a millivolt gravity system can produce a trickle of heat without power, but that design is rare and does not apply to anything with a blower and a circuit board.
This is the opposite of the electric-furnace situation. An electric furnace in an outage is not a small load you back up; it is 10 to 20 kW that no realistic home battery or portable generator can carry, which is why homes that heat with electricity usually plan outage heat around a wood stove, a few space heaters on a generator, or not freezing rather than running the furnace.
What size generator or battery do I need to run a furnace?
For a gas furnace, a generator or power station rated for about 1,500 to 3,500 watts is plenty, and the extra headroom is there for the blower's startup surge rather than its steady draw. A 2,000 to 3,000-watt inverter generator runs a gas furnace with room to spare, and it can usually carry a refrigerator and some lights at the same time. On battery, a gas furnace's small load means a 1 to 2 kWh power station can run the blower for roughly 1.5 to 4 hours, and a larger 2 to 5 kWh unit gets you through a long evening; our what a 100Ah battery can run breakdown shows how that math works for the blower specifically.
For an electric furnace, you need a 12 to 25 kW standby generator, not a portable. The rule of thumb is to size the generator above the furnace's kW rating with margin: a 10 kW furnace wants a generator rated around 12 kW, and a 20 kW furnace wants one near 22 to 25 kW. That is a permanently installed, professionally wired standby unit, the same class of machine it takes to back up a whole all-electric house. A portable generator will trip or stall the instant the resistance strips call for full power.
Whichever furnace you have, read its rating plate before buying anything. The furnace's FLA tells you the running amps, and multiplying amps by volts gives watts; then add the blower surge for a gas unit, or simply size well above the kW rating for an electric one. Do not size a generator from the breaker, because a 15-amp breaker protects a circuit rated to 1,800 watts regardless of what the furnace actually draws.
Can you run a furnace on solar or a battery?
A gas furnace is an excellent solar and battery backup load; an electric furnace is not. The gas furnace's 400 to 800 watts is small, intermittent (the blower only runs during a call for heat), and easy to cover with a modest battery topped up by panels. In a grid-tied home, the furnace blower is a trivial slice of your annual usage; in an outage, a solar generator or a battery with some panels can keep it cycling through a storm. Size the whole picture with the solar panel calculator and let net metering balance the hours the sun is not up.
An electric furnace is one of the worst possible loads for solar or batteries. At 10 to 20 kW it would need a very large array and a wall of batteries just to cover the hours it runs, and it runs hardest in winter when solar production is at its lowest. A home that heats with an electric furnace and wants to lean on solar is almost always better off replacing the furnace with a heat pump first, which cuts the heating load to a third, and only then sizing the array. See the heat pump wattage guide for how that math changes.
So the honest framing is that backing up heat is easy if you burn gas and hard if you heat with electricity. For a gas furnace, a solar generator or a battery with panels is a sensible winter-outage plan. For an electric furnace, plan backup heat around a different source, and treat solar as a reason to switch to a heat pump rather than a way to feed the resistance strips.
Frequently asked questions
Can a gas furnace run without electricity?
No. Even though the heat comes from burning gas, a modern gas furnace needs electricity for the blower motor that moves warm air, the igniter that lights the burner, the control board that runs the cycle, and the gas valve that is held open electrically and closes the moment power is lost. Cut the power and the furnace shuts down. The good news is that the running draw is only about 400 to 800 watts, so a mid-size battery, a solar generator, or a small inverter generator can keep a gas furnace running through an outage.
What size generator do I need for my furnace?
For a gas furnace, a 2,000 to 3,500-watt generator is plenty, with the extra capacity there to absorb the blower's startup surge rather than its 400 to 800-watt running draw. A 2,000 to 3,000-watt inverter generator runs a gas furnace plus a fridge and lights comfortably. For an electric furnace, you need a 12 to 25 kW standby generator: size it above the furnace's kW rating, so a 10 kW furnace wants about 12 kW and a 20 kW furnace wants 22 to 25 kW. Read the furnace's rating plate FLA to confirm, and never size a generator from the breaker amperage alone.
Do electric furnaces use a lot of electricity?
Yes, a great deal. An electric furnace uses about 10,000 to 20,000 watts (10 to 20 kW) while heating, because it makes heat by resistance, exactly like a giant space heater in your ductwork, with a coefficient of performance of 1.0. A 20 kW unit running flat out uses 20 kWh every hour, which can double or triple an all-electric home's daily electricity use in the coldest months. That is why a heat pump, which moves heat and delivers three to four units of heat per unit of electricity, costs roughly a third as much to run for the same warmth.
Can I plug my gas furnace into a generator?
You can power a gas furnace from a generator, but not by running an extension cord into it, because a furnace is hardwired into the service panel. The safe way is a transfer switch (a manual interlock kit or an automatic transfer switch) installed by an electrician, which isolates the furnace circuit from the utility and lets the generator feed it without backfeeding the grid. Backfeeding through a dryer cord or a double-male cable is dangerous and illegal: it can electrocute a lineworker and start a fire. Once a transfer switch is in place, a 2,000 to 3,500-watt generator runs a gas furnace easily.
Can you run an electric furnace on a generator?
Only on a large standby generator, not a portable. An electric furnace draws 10 to 20 kW of pure resistance heat, so it needs a permanently installed 12 to 25 kW standby unit sized above the furnace's rating, professionally wired through a transfer switch. A typical 3,000 to 8,000-watt portable generator will trip or stall the instant the heating elements call for full power. If outage heat matters and you have an electric furnace, it is usually more practical to back up a couple of space heaters or a wood stove than to try to run the whole furnace.