What Size Generator for a Refrigerator and Furnace?
Updated May 1, 2026
Just want the number? Use the generator size calculator.
A refrigerator plus a gas furnace blower plus some lights usually runs on a 3,500 to 5,000 watt generator, once you account for the startup surge that the fridge and blower demand when they kick on. The steady draw of those items is modest, but a generator has to handle the brief surge too, and that surge is what pushes you toward the larger end of the range.
Here is how to get to a number you can trust.
Running watts vs starting watts
Every motor-driven appliance has two ratings, and confusing them is the most common sizing mistake.
- Running watts is the steady power the appliance pulls once it is up and going.
- Starting watts, also called the surge, is the brief spike a motor pulls at the instant it switches on. It lasts a second or two but can be two to three times the running figure.
A generator must cover both. You add up the running watts of everything that will be on at the same time, then add the single largest startup surge on top, since only one motor tends to start at any given moment.
The two loads you care about
For a fridge-and-furnace setup, these are the figures to work with. Check your own nameplates, because models vary.
- Refrigerator: about 700 running watts, roughly 2,200 watts to start
- Gas furnace blower: about 800 running watts, roughly 2,350 watts to start
- Lights and small electronics: about 200 to 400 running watts, no real surge
Notice that the running numbers are small while the surge numbers are not. That gap is the whole reason a generator that looks big enough on paper can stall the moment the compressor or blower cycles on.
Adding it up
Walk the math one step at a time.
- Total the running watts of everything on together. Fridge (700) plus furnace blower (800) plus lights (300) gives about 1,800 running watts.
- Find the single largest surge. Here the furnace blower wins at about 2,350 starting watts.
- Add the largest surge to the running total of the other items. So 1,800 running, minus the blower’s 800 running, plus the blower’s 2,350 surge, lands at a peak of roughly 3,350 watts.
That peak, not the 1,800 running total, is what the generator has to deliver. Size to the steady load alone and the unit will trip the instant a motor starts.
Pick a size with margin
A peak near 3,350 watts means a 3,500 watt generator is cutting it close with nothing to spare. Add about 10 to 20 percent headroom so the unit is not pinned at maximum, and a 4,000 to 5,000 watt generator is the comfortable pick. That margin also leaves room to plug in a phone charger or a sump pump without redoing the math. For a broader look at what a full house needs during an outage, what size generator to run a house covers the complete appliance list with wattage figures for a sump pump, well pump, and more.
The generator size calculator totals the running watts, finds the biggest surge, and adds headroom for you, with the math shown.
Gas furnace vs electric furnace
This matters a lot. The figures above assume a gas (or oil) furnace, where the only electrical load is the blower motor and the controls. An electric furnace is a different animal. Its heating elements draw power directly, often 10,000 to 20,000 watts, which is far more than a typical portable generator can supply.
If you heat with an electric furnace, a small generator will run the blower and the controls but not the heat itself. Most people in that situation back up the fridge and lights, then heat with a separate gas or propane source during an outage. To see what your heating system actually pulls, the furnace size calculator and the energy cost calculator can give you the numbers before you decide what to back up.
A safety note
Run any fuel-burning generator outdoors and well away from windows and doors. Carbon monoxide from a generator placed in or near the house is a serious hazard, and the U.S. Consumer Product Safety Commission publishes placement guidance worth reading before you need it.
List the items you want to keep running, total their running watts, add the largest single surge, and add headroom on top. Run your own loads through the generator size calculator for a sized recommendation with the surge math included.