What Size Generator for a Furnace?
A 3,400W inverter runs a gas furnace: the burner needs almost nothing, but the 1/2 HP blower motor draws about 800W and starts at 2,350. The catch isn't the watts, it's the wiring: furnaces are hardwired, so getting generator power into one takes an interlock, a transfer switch, or an electrician-installed furnace plug.
Wattage by type
| Furnace type | Running | Starting |
|---|---|---|
| ECM variable-speed blower | 250 to 400 W | 400 to 700 W |
| 1/3 HP PSC blower | 500 W | 1,500 W |
| 1/2 HP PSC blower (planning number) | 800 W | 2,350 W |
| 3/4 HP PSC blower | 1,000 W | 2,900 W |
| Boiler (circulator pumps + controls) | 150 to 400 W | 400 to 800 W |
What it takes with company
Same engine as the wattage picker: running total plus the single biggest start spike, then the smallest class that carries it with 20% headroom.
| Load list | Running | Peak | Buy |
|---|---|---|---|
| Furnace alone | 800 W | 2,350 W | 3,400W inverter |
| Furnace + fridge + the basics | 1,700 W | 3,250 W | 3,400W inverter |
| Furnace + fridge + sump + the basics | 2,750 W | 4,300 W | 3,500W portable |
The details that change the answer
Heat in a winter outage is the highest-stakes item on the whole site, and the watt math is the easy part: gas and oil furnaces burn fuel for heat and only need electricity for the blower, the controls, and the igniter. An older PSC blower motor is a normal induction motor with a normal triple-ish start spike. A modern ECM motor ramps softly and cuts both numbers roughly in half, which is worth knowing before you buy two sizes of generator you don't need.
The wiring is the hard part. You cannot unplug a furnace, so the choices are: an interlock kit and inlet on the panel (an electrician morning, and the whole panel becomes usable), a small transfer switch on the furnace circuit, or having the furnace converted to plug-and-receptacle so it can meet an extension cord. All three are legitimate; the extension cord through the window to a hardwired furnace is not, and neither is any arrangement that could feed the grid. The transfer switch guide walks the options.
Boilers are the quiet winners here: circulators and controls, a few hundred watts, no surge worth naming. If you heat with hot water, a 2,200W inverter carries the whole heating system plus the fridge.
Questions people ask
Will a 2,000W generator run my furnace?
If the blower is a 1/3 HP or an ECM variable-speed motor, yes: worst case there is a 1,500W spike against a 2,200W peak rating. A 1/2 HP PSC blower starts at about 2,350W and will stall the class; that furnace wants a 3,400W unit. The motor label inside the blower door says which you have, and an electric-resistance furnace is a different animal entirely (10kW+, standby territory).
Does a heat pump work on a portable generator?
The blower does; the compressor mostly doesn’t make sense. A 3 ton heat pump compressor is central-AC math (3,500 running, 6,000 starting), and its backup resistance strips are 5 to 15 kW on top. In an outage, run the air handler for circulation if it helps, but plan real heating around fuel: gas furnace, wood, or a properly vented propane heater.