A battery with plenty of watt-hours will still refuse to start an older blower motor. Our sizing guide walks through surge, continuous output and usable capacity.
Size a power station properly →Running a gas furnace on a battery is one of the highest-value moves in a winter outage, and it is far more achievable than most people assume. The gas keeps flowing when the grid drops. What stops is the blower, the draft inducer and the control board — a few hundred watts of electricity standing between your family and a house that cools off at about a degree an hour. This is how much power the furnace really needs, and how big a battery has to be to carry it through the night.
Why the Furnace Dies When the Gas Is Still There
A modern gas furnace is a sequenced machine, and every step in the sequence is electric. The thermostat calls. The control board wakes up and runs a safety check. The draft inducer motor spins up to prove there is a path out the flue, and a pressure switch confirms it. Only then does the hot surface igniter glow, the gas valve opens, the flame sensor confirms burning, and the blower motor starts moving air.
Take away 120-volt power and the board never wakes up, the inducer never spins, the pressure switch never closes, and the gas valve stays shut. That last part is a feature. A furnace that dumped gas into a heat exchanger with no air moving over it would crack the exchanger and push combustion products into the house. The interlocks are doing exactly what they were designed to do.
What the Furnace Actually Draws
The single biggest variable is the blower motor. Older permanent split capacitor (PSC) motors run at one or two fixed speeds and pull a hard inrush at startup. Newer electronically commutated (ECM) motors ramp up smoothly and use dramatically less power. Look at the data plate on the furnace door, or the nameplate on the blower itself — the difference between the two is the difference between a 1,000 Wh battery lasting five hours and lasting twelve.
| Component | Running watts | Startup / peak watts |
|---|---|---|
| PSC blower motor | 200–800 | 400–2,000 |
| ECM blower motor | 50–400 | 100–800 |
| Draft inducer motor | 100–300 | 200–800 |
| Hot surface igniter | 500–1,200 for ~20–40 seconds | same |
| Control board and transformer | tens of watts | negligible |
Those ranges are wide because furnaces are. The practical takeaway: a PSC furnace can ask for something on the order of 2,500 to 3,000 watts for a second or two during the worst part of a start, and settle at roughly 500 to 700 watts while it burns. An ECM furnace often settles nearer 150 to 300 watts. Measure yours with a clamp meter if you can, or work it out from the nameplate amps; the method is the same one in our home energy audit.
Watt-Hours Are the Real Question
Watts tell you whether the inverter can start the thing. Watt-hours tell you how long the battery lasts. A furnace does not run continuously — it cycles. In a genuinely cold snap, a correctly sized furnace might run 20 to 30 minutes out of every hour, call it a duty cycle of a third to a half. In milder weather it might be a tenth.
Multiply running watts by duty cycle to get average draw:
- PSC furnace, 600 W while running, one-third duty: about 200 Wh per hour, so roughly 2.4 kWh over twelve hours.
- ECM furnace, 250 W while running, one-third duty: about 83 Wh per hour, so roughly 1 kWh over twelve hours.
- Same ECM furnace in a mild outage, one-tenth duty: about 25 Wh per hour — a small battery covers a whole day.
Against a power station with 2,000 Wh nominal and maybe 1,800 Wh usable, that ECM furnace runs most of a full day and the PSC furnace runs somewhere around eight or nine hours. Both figures assume the furnace is the only thing plugged in, which is the assumption most people quietly break.
The Four Things That Wreck the Estimate
Inverter surge capability. A unit rated for 1,000 W continuous with 2,000 W surge will start many ECM blowers and stall on a tired PSC one. If the battery trips its overload protection on every furnace start, capacity is irrelevant.
Standby losses. Most inverters burn something like 5 to 20 watts just being switched on. Over 24 hours that is 120 to 480 Wh gone before the furnace asks for anything. Some units let you disable an always-on display or a wireless radio; do it.
Waveform. ECM motors and furnace control boards want a true sine wave. Modified sine inverters can make boards behave erratically or refuse to run at all, and they run motors hot. This is not a place to save money.
Cold. Lithium iron phosphate cells lose capacity in the cold and most will not accept a charge below freezing. A battery parked in an unheated garage in January delivers noticeably less than its label. Keep it in the conditioned space, which is where the furnace is anyway.
How You Connect It Matters
Most residential furnaces are hardwired to a dedicated circuit through a service switch that looks like a light switch on the side of the unit. You cannot simply unplug it. There are three legitimate options, and one that gets people killed.
- Have an electrician install a cord and receptacle at the furnace, so it can be unplugged from house power and plugged into the battery. Cheap, and the most common approach.
- Have an electrician install a small manual transfer switch that feeds the furnace circuit from an inlet. Cleaner, more expensive, no crawling around with cords. See transfer switch types and cost.
- Feed the whole critical-loads panel from a larger battery or generator, which is a bigger project and a different budget.
Safety: Never back-feed your house through a dryer outlet or a double-male extension cord. It energizes the utility line outside, it bypasses every interlock in the panel, and it kills line workers and homeowners. Never run a fuel-burning generator indoors or in an attached garage, even with the door open — carbon monoxide from a generator can be lethal within minutes.
The Honest Verdict
If your furnace has an ECM blower, a mid-size power station in the 1,500 to 2,000 Wh class with at least 2,000 W of surge is a genuinely good answer for an overnight outage, and it does the job silently, indoors, with no fuel to store. If you have an older PSC blower, be realistic: you are looking at a bigger unit or an expansion battery, and you will still be measuring runtime in hours rather than days. Either way, keep the furnace on its own — the temptation to also run the refrigerator off the same battery is exactly what turns a twelve-hour plan into a five-hour one. And whatever you buy, plug the furnace into it on a mild evening and watch it complete three full cycles before you need it in earnest.
Frequently Asked Questions
How many watts does a gas furnace need to run?
Typically 150 to 300 watts continuous for a furnace with an ECM blower, and 500 to 700 watts for one with an older PSC blower. Startup can briefly hit 2,500 to 3,000 watts on a PSC unit because of blower inrush plus the inducer and igniter.
Will a 1000 watt power station run my furnace?
Often yes for an ECM furnace, if the surge rating is at least 2,000 W. For a PSC blower it is marginal — it may start on a warm day and fail on a cold one when the motor is working harder. Check the surge spec, not just the continuous rating.
Can I run my furnace on a car battery and an inverter?
Briefly, and badly. A typical starting battery holds a few hundred usable watt-hours before deep discharge damages it, and lead-acid chemistry hates being drawn down. A deep-cycle or lithium battery with a true sine wave inverter is the right tool.
Does a battery-run furnace still need the thermostat?
Yes, and it works normally. The thermostat runs on 24 volts from a transformer inside the furnace, so once the furnace has power, the thermostat has power. Battery-backed smart thermostats still need the furnace energized to do anything.
How cold does a house get without heat?
It depends entirely on insulation, wind and outdoor temperature, but a reasonably tight house commonly drops something like one degree Fahrenheit per hour in a hard freeze. That is slow enough to sleep through and fast enough to matter by the second night — for pipes as much as for people.