What size generator do I need to run a gas furnace?
Short answer: A gas furnace's ½ hp blower runs on about 800 W but needs about 2,350 W for a second as it starts. With the fridge, lights, internet and phones on too, that is 1,225 W running and 2,775 W peak: a 3,500 W inverter generator covers it. Add a ½ hp sump pump and you need a 5,000 W portable; a 1 hp, 240 V well pump takes it to a 7,500 W portable.
Generator size by what you keep on
The burner in a gas furnace uses very little electricity. The control board, gas valve and inducer fan draw a few hundred watts at most, and a hot-surface igniter adds a few amps for less than a minute. What sizes the generator is the blower motor that pushes warm air through the ducts. Our calculator uses 800 W running and 2,350 W starting for a ½ hp blower, then adds the other loads a winter outage usually needs, with motors started one at a time:
2,200 W inverter3,500 W inverter7,200 W: a 30 A (L14-30) inlet
| What is running | Running W | Peak W | Smallest common generator | Typical connection |
|---|---|---|---|---|
| Furnace blower only | 800 | 2,350 | 3,500 W inverter | TT-30R (30 A, 120 V) |
| Furnace + fridge, lights, internet, phones | 1,225 | 2,775 | 3,500 W inverter | TT-30R (30 A, 120 V) |
| Same + ½ hp sump pump | 2,275 | 3,825 | 5,000 W portable | L14-20 or L14-30 |
| Same + microwave (sump pump on) | 3,275 | 4,825 | 7,500 W portable | L14-30 (30 A, 120/240 V) |
| Furnace essentials + 1 hp well pump | 2,725 | 5,225 | 7,500 W portable | L14-30 (30 A, 120/240 V) |
| Furnace essentials + sump + well pump | 3,775 | 6,275 | 7,500 W portable | L14-30 (30 A, 120/240 V) |
Two things stand out. The blower alone peaks at 2,350 W, just above what a 2,200 W inverter generator can surge, so a small inverter is a gamble unless your blower is smaller or soft-starting (see below). And a 1 hp well pump is a 240 V load, which rules out most 3,500 W inverters: they only have 120 V outlets, so the well pump pushes you to a 7,500 W portable with a 120/240 V outlet.
Read your blower, not the box
Open the blower compartment door and look for the motor's label. Two things matter:
- Horsepower and FLA (full-load amps). Running watts ≈ FLA × 120 V. Blowers range from ⅓ hp in small furnaces to 1 hp in large ones.
- Motor type. Older furnaces use a single-speed PSC motor, which surges when it starts. Many newer furnaces use a variable-speed ECM motor that ramps up gradually and draws less while running. With an ECM, the starting surge is much smaller.
Put your own numbers in the calculator: once the furnace is selected, its running and starting watts are editable. A hot-water boiler is a different case: its circulator pump is far smaller than a furnace blower, so the control board and igniter set the load.
Why inverter generators suit furnaces
Furnace control boards and ECM motors are electronic, and they don't like the voltage and frequency swings of a conventional open-frame generator under changing load. An inverter generator produces a clean, steady sine wave, which is why HVAC technicians usually recommend one for a furnace. It also throttles down at light load, and a furnace plus the fridge is a light load.
A common problem: the furnace starts, the burner lights, then it shuts off after a few seconds and retries. Furnaces prove the flame through a small current that needs the neutral and ground bonded together. Many portable generators have a "floating" neutral, so a furnace plugged straight into one can fail to sense its own flame. Connected through a transfer switch or interlock that uses your panel's bond, this usually goes away. If it doesn't, an electrician can tell you whether a bonding plug is appropriate for your setup.
Getting power to a hardwired furnace
A furnace is hardwired to its own 120 V circuit, with a service switch near the unit, so there is no plug to move to an extension cord. The options:
- Interlock kit or manual transfer switch on the main panel, fed from an inlet box outside. This powers the furnace and the other circuits you need. See interlock kit vs transfer switch.
- Single-circuit furnace transfer switch, installed between the furnace circuit and the furnace, with an inlet for a generator cord. It is cheaper but powers only the furnace.
Cutting the furnace's wiring and adding a plug is sometimes allowed by local code and sometimes not; ask your inspector before doing it.
How much gas for a cold night
Gasoline use from our fuel calculator, running each setup continuously on the generator size above:
| What is running | Generator | Load | Gallons per hour | Per 24 h | For 3 days |
|---|---|---|---|---|---|
| Furnace + fridge, lights, internet, phones | 3,500 W inverter | 41% | 0.29 | 7.0 | 21 |
| Same + ½ hp sump pump | 5,000 W portable | 57% | 0.43 | 10.3 | 31 |
| Same + microwave (sump pump on) | 7,500 W portable | 55% | 0.63 | 15.1 | 45 |
| Furnace essentials + 1 hp well pump | 7,500 W portable | 45% | 0.56 | 13.5 | 40 |
These are upper limits. The blower cycles with the thermostat and the fridge compressor rests, so the real average load is lower. Turning the thermostat down a few degrees and closing off unused rooms cuts blower run time. Either way, 21 gallons for three days is more gas than most people store; standby units on propane or natural gas avoid the problem. See how much propane a standby generator uses.
Size it for your furnace and loads
Safety. The CPSC links more than 100 U.S. deaths a year to carbon monoxide from portable generators. Never run one inside the house or garage, even with doors and windows open. Keep it at least 20 feet from the house with the exhaust pointed away, clear of snow, and away from your furnace's own intake and vent pipes. Put battery or battery-backup CO alarms on every level of the home. See generator CO safety.