What Size Generator for Furnace/Boiler?
Gas and oil furnaces still need electricity for the blower, igniter, and controls. A 2500W generator runs most furnaces comfortably. Electric furnaces and heat pumps need much larger generators.
Minimum Size
2000W
bare minimum, tight
Comfortable Size
3000W
recommended
Ideal Size
5000W
room to grow
Total Running Watts
500W
Total Surge/Starting Watts
1500W
Generator Sizing Calculator
Additional Custom Load
Running Watts
1500W
Surge Watts
4400W
Recommended Generator
5500W+
Includes 25% headroom for reliable operation
Suitable Generator Sizes
Estimated Fuel Usage
* Fuel estimates are approximate. Actual consumption varies by generator model, load, and altitude.
Typical Loads for Furnace/Boiler
| Item | Running Watts | Surge Watts |
|---|---|---|
| Gas Furnace (blower) | 500W | 1500W |
| Oil Furnace (blower + burner) | 700W | 2000W |
| Boiler (circulator pumps) | 200W | 400W |
| Pellet Stove | 100W | 500W |
| Total | 500W | 1500W |
* Surge watts apply to motor loads during startup. Size your generator for worst-case surge scenario.
Generator Sizing Tips
- •Gas/oil furnaces only need power for blower and controls, not heat
- •The blower motor surge is the biggest challenge
- •Electric space heaters are NOT practical on generators (1500W each)
- •Pellet stoves are very generator-friendly (low power)
- •A wood stove needs no electricity at all
Key Considerations
- ⚠Carbon monoxide risk - keep generator outside, vented properly
- ⚠Thermostat and controls also need power
- ⚠Extension cords to furnace must be properly sized
Alternatives to Consider
- ✓Wood stove or fireplace (no electricity needed)
- ✓Propane/kerosene space heater (use with ventilation)
- ✓Pellet stove (low power needs)
Frequently Asked Questions
What size generator for a gas furnace?
Most gas furnaces need 2000-2500W to handle the blower motor surge. Running watts are only 300-700W. A 3000W generator is comfortable for a furnace plus a few other loads.
Can I run my furnace on a portable generator?
Yes, but you need a transfer switch or to run an extension cord directly to the furnace (if accessible). Never backfeed through an outlet. A 3000W inverter generator works well.
What about electric furnaces or heat pumps?
Electric furnaces and heat pump backup strips need 10,000-15,000W - not practical for portable generators. Use the heat pump without backup heat (3000-5000W) or use an alternative heat source.