What a furnace actually needs electricity for
This is the insight that makes winter backup affordable. A gas or oil furnace does not use electricity to make heat — it burns fuel for that. Electricity runs four much smaller things.
- The blower motor, which moves warm air through the ducts. This is nearly all of the load.
- The control board and thermostat, which are a few watts.
- The igniter, briefly, at the start of each cycle.
- The draft inducer fan, which clears the flue before ignition.
So the question "can I run my furnace on a generator" is really "can I run a blower motor", and the answer is almost always yes.
The numbers
| System | Running watts | Notes |
|---|
| Gas furnace, blower and controls | Up to about 1,600 W | ECM blowers draw considerably less than older PSC ones |
|---|
| Oil furnace | Similar, plus a fuel pump | The burner motor adds a little |
|---|
| Boiler with circulator pumps | Often only 100-300 W | The cheapest heat to back up in the whole category |
|---|
| Heat pump | Thousands of watts | This is not a blower load — it is a compressor |
|---|
| Electric resistance furnace | 10,000 W and up | No portable machine runs this |
|---|
Blower figures are the ranges used across this site; see the furnace power page for the detail and how to find your own. The plate inside the furnace door gives the authoritative figure for your unit.
The bottom two rows are the important caveat. If your heat is electric — a heat pump or a resistance furnace — none of the good news on this page applies, and you are in a very different and much more expensive conversation.
How long a battery runs a blower
A furnace blower cycles rather than running continuously. On a cold night a well-insulated house might call for heat something like fifteen minutes in every hour; a poorly insulated one in severe cold, considerably more.
| Duty | Average draw at 800 W blower | Hours from 2,048 Wh |
|---|
| 15 min in 60 | 200 W | About 8.7 hours |
|---|
| 25 min in 60 | 333 W | About 5.2 hours |
|---|
| 40 min in 60 | 533 W | About 3.3 hours |
|---|
Calculated as 2,048 Wh x 0.85 / average draw. Blower draw and duty cycle vary widely — an 800W ECM blower is used here as a mid-range example.
The pattern: a 2,000Wh battery covers most of a night in moderate cold and a few hours in severe cold. That is genuinely useful, and it is silent and indoors.
Getting power to it
A furnace is hardwired, so there are three routes and they differ a lot in cost.
1. A transfer switch. The full answer: reaches the furnace and everything else hardwired, permanently installed, safe by construction. The transfer switch page covers sizing.
2. An interlock kit. Cheaper, uses your existing panel, and legal where the panel manufacturer lists a kit for it — the comparison page covers the trade.
3. A cord-and-plug conversion at the furnace. An electrician fits a receptacle and a short cord so the furnace can be unplugged from house power and plugged into an extension cord from a generator or battery. Much cheaper, furnace-only, and subject to local code.
What you must never do is backfeed the panel through a dryer outlet with a double-ended cord. It is illegal everywhere in the United States and it is the specific hazard that kills lineworkers.
Run it in blocks
Houses hold heat. Running the furnace for an hour to bring the house up several degrees and then letting it coast is far more efficient than holding a constant temperature, and it cuts generator fuel use dramatically.
Close off rooms you are not using, keep the thermostat lower than normal, and accept a cooler house. The staying warm page covers the non-electrical half of the same problem.
The short version
- A gas furnace needs power for the blower, not the heat. Up to about 1,600W.
- A boiler is cheaper still — often 100 to 300W of circulator pumps.
- Heat pumps and electric furnaces are a different problem and no portable machine solves them.
- It is hardwired, so budget for a transfer switch, interlock, or a cord-and-plug conversion.
