Quick answer: There is no single furnace blower watts figure. Trane publishes 27 W to 499 W for the blower in one current 40,000 BTU furnace, depending on the speed setting and how hard your ducts push back. At typical duct pressure it is 90 W on the lowest heat setting and 417 W on the highest.
The heat itself is not your problem in an outage: the gas is still there. What stops is everything electrical around it, and by far the largest of those is the fan that moves the warm air through the house. So the question is mostly, though not entirely, about one motor.
Most answers give you a single figure. The manufacturers publish something far more useful: a full grid of watts against airflow and duct resistance. Our appliance wattage chart has to compress that into one range; here is the grid it comes from.
Furnace blower watts: what a manufacturer actually publishes
These are Trane’s own figures for the S9V2B040U3PSBB, a current 96 % AFUE two-stage furnace with a variable-speed motor, taken from its product data sheet. The watts are measured with a filter fitted, not calculated from amps.
| Heating setting | CFM at 0.5 in. | Watts at 0.1 in. | Watts at 0.5 in. | Watts at 0.9 in. |
|---|---|---|---|---|
| 1st stage, low | 437 | 27 | 90 | 152 |
| 1st stage, medium low | 647 | 41 | 112 | 183 |
| 1st stage, medium | 687 | 48 | 118 | 189 |
| 1st stage, high | 879 | 125 | 232 | 339 |
| 2nd stage, low | 639 | 48 | 135 | 223 |
| 2nd stage, medium low | 813 | 82 | 182 | 282 |
| 2nd stage, medium | 842 | 94 | 189 | 284 |
| 2nd stage, high | 1,200 | 335 | 417 | 499 |
The lowest published figure is 27 W. The highest is 499 W. That is a spread of eighteen times, from a single furnace, and any article giving you one number has quietly picked a point on this grid without telling you which.
Your ductwork is half the answer
Read the table across rather than down and something uncomfortable appears. Nothing about the furnace changes between those columns. Only the resistance of the duct system does.
On the lowest heat setting, moving from easy ducts to restricted ones takes the blower from 27 W to 152 W. A variable-speed motor works to hold the airflow it was asked for, so when the ducts fight back it simply pushes harder and draws more.
The practical version: a clogged filter is not just a comfort problem during an outage, it is a battery problem. It is also the cheapest thing on this entire site to fix.
The date on your furnace decides which motor you have
The figures above are for an electronically commutated motor, an ECM. If your furnace is older it may have a permanent split capacitor motor instead, and that changes the arithmetic considerably.
There is a hard date. The US Department of Energy set an efficiency metric for furnace fans called the Fan Energy Rating, measured in watts per 1,000 cubic feet per minute. Lennox puts the consequence plainly in its own dealer guidance: PSC motors cannot meet the power consumption requirement, so after 3 July 2019 furnaces may only be manufactured with an ECM.
Two things follow that are worth knowing before you size anything:
- A furnace built after July 2019 has an ECM. Its blower figures look like the table above.
- Stock built before that date could still be sold and installed. Lennox says so explicitly, so a furnace installed in 2020 or 2021 is not automatically an ECM. The build date decides it, not the installation date.
- Air handlers used with heat pumps are not covered by the rule at all, and neither are the small inducer fans.
The blower is not the only thing drawing power
Whenever the furnace fires, a second motor runs: the combustion inducer that pulls exhaust through the heat exchanger. Trane rates it at 0.66 A at 120 V, so budget something under 80 VA for it, and remember it is running before the blower starts and after it stops.
There is also a hot surface igniter, which draws hard for a few seconds at each start, and the control board. None of these is large. All of them are additional to the blower figure, which is the one every chart quotes on its own.
What furnace blower watts mean when you are sizing backup
A furnace blower behaves very differently from the pumps elsewhere in this silo, and mostly in your favour.
- You can choose the setting. At normal duct pressure the difference between the lowest and highest heat setting is 4.6 times. A well pump gives you no such choice.
- There is no violent surge. Trane publishes no locked-rotor figure for these blowers, because an ECM is ramped up electronically rather than thrown across the line, so the usual starting-watts penalty barely applies. That is the opposite of the sump and well pumps, where the startup number decides everything.
- Do not size to the nameplate amps. The blower motor is rated 5.7 A at 120 V, an apparent-power ceiling of 684 VA, yet the highest figure Trane actually publishes is 499 W. As always, amps times volts is a ceiling, not a draw.
- Ignore the minimum circuit ampacity. Goodman prints the formula on its own spec sheet: minimum circuit ampacity is 1.25 times the blower amps plus the inducer amps. It is a wire-sizing number with a safety margin built in, and it will always overstate what the furnace consumes.
In energy terms this is a modest load. Twelve hours on the lowest setting at normal duct pressure is about 1.1 kWh, roughly 20 US cents. Twelve hours on the highest is 5 kWh, near 92 cents. For a home battery, the difference between those two numbers is the difference between getting through the night and not.
Safety. A furnace is a combustion appliance and it is hard-wired. Do not open the cabinet to read a label, do not improvise a connection to a generator, and do not run the furnace with any part of its venting disturbed. This article describes what the equipment draws; the wiring belongs to a licensed electrician and the gas side to a licensed heating engineer.
If you intend to run a furnace during an outage, the connection is a transfer switch installed under NEC Article 702, and the generator lives outside, well away from windows, doors and vents. Carbon monoxide from a portable generator kills quickly and without warning, and a furnace is already a reason to have working CO alarms. In the UK this work falls under BS 7671 and Gas Safe registration.
Frequently asked questions
How many watts does a gas furnace use during a power outage?
Only what the blower and the small combustion fan draw; the heat itself is gas. For a current variable-speed furnace Trane publishes 27 W to 499 W for the blower, plus under 80 VA for the inducer.
Does my furnace have an ECM or a PSC blower?
If it was manufactured after 3 July 2019 it has an ECM, because a PSC motor cannot meet the federal Fan Energy Rating. Older units may have either. Check the build date on the rating plate rather than the year it was installed, since pre-2019 stock could still be sold.
Will a 2,000 W generator run my furnace?
On the published figures the furnace itself is a small load: even at its highest setting and worst duct pressure the blower is 499 W, and the inducer adds under 80 VA on top. The limit is usually not generator capacity but the connection: running a hard-wired furnace from one legally requires a transfer switch fitted by an electrician.
Why do different sources give such different furnace wattages?
Because they are quoting different points on a grid without saying so. One furnace here spans 27 to 499 W across its speed settings and duct pressures, and older PSC blowers sit higher again. A single number for “a furnace blower” cannot be right for long.
Does a dirty filter really change the electricity?
Yes, and more than most people expect. A restricted system raises static pressure, and on the lowest heat setting Trane’s own table shows the blower going from 27 W to 152 W as pressure rises. The motor is holding airflow by working harder.
This article is general information, not an installation guide and not a substitute for a licensed electrician. Electrical work on a service panel, a transfer switch or a utility connection is regulated in most jurisdictions and normally requires a permit and an inspection. Fuel-burning equipment produces carbon monoxide and must never be run indoors, in a garage or near an opening into the home.
