What Size Generator Do You Need for a Furnace or Boiler?
A gas or oil furnace still needs electricity: the blower fan, igniter and controls stop without power. The blower is a modest motor, so keeping the heat on in a winter outage takes far less generator than most people expect.
| Appliance | Running / starting W | On its own | With fridge, lights and Wi-Fi |
|---|---|---|---|
| Gas furnace blower (1/3 HP) | 700 / 1,400 | 2,000-watt 10 kW standby | 3,000-watt 10 kW standby |
| Gas furnace blower (1/2 HP) | 875 / 2,350 | 3,000-watt 10 kW standby | 3,000-watt 10 kW standby |
| Boiler, gas or oil (burner + circulator) | 300 / 700 | 2,000-watt 10 kW standby | 3,000-watt 10 kW standby |
Smallest portable (by starting watts) and smallest standby generator on natural gas, keeping about 20% headroom.
Tips
- Furnaces are sensitive to unstable power; an inverter generator or a standby unit gives cleaner electricity.
- Many furnaces are hard-wired: an electrician can add a transfer switch or a plug-in cord kit so a portable can run it safely.
- Electric furnaces and heat-strip backup are a different story: they need 10–20 kW, often more than the rest of the house.
See also the appliance wattage chart and what each generator size can run.