What Size Generator Do I Need to Run My House?
Updated April 27, 2026
Just want the number? Use the generator size calculator.
Most homes get through an outage on a 5,000 to 7,500 watt generator that covers the essentials (fridge, furnace blower, a sump or well pump, some lights and outlets), while running nearly the whole house at once usually takes 10,000 watts or more. Where you land inside that range depends less on your square footage and more on which motors you need to start at the same time.
Running watts vs starting watts
Every spec sheet lists two numbers, and mixing them up is the most common sizing mistake.
- Running watts (also called rated watts) is the steady draw an appliance pulls once it is up and going.
- Starting watts (the surge) is the brief spike a motor demands at the instant it kicks on. For a fridge, furnace blower, or pump, the surge can be two to three times the running figure for a second or two.
A generator has to supply both. You add up the running watts of everything that will be on together, then add the single largest starting surge on top, because only one motor tends to start at any given moment. That combined peak, not the running total, is what sets the size you need.
Typical appliance wattages
Rough figures for common backup loads. Check your own nameplates, since models vary.
- Refrigerator: about 150 running watts, roughly 800 to 1,200 to start
- Furnace blower (gas or oil): about 600 to 800 running, 1,500 to 2,300 to start
- Sump pump: about 800 to 1,050 running, 1,300 to 2,150 to start
- Well pump (1/2 to 1 HP): about 1,000 to 2,000 running, 2,000 to 4,000 to start
- Lights and small electronics: about 60 to 600 running total, no real surge
- Window AC unit: about 1,000 to 1,500 running, 2,200 or more to start
Add the running watts of the items you cannot do without, then tack on whichever of those has the biggest surge. The generator size calculator does this for you and shows the math.
Why the largest motor sets your peak
Say your running total comes to 3,500 watts and your well pump has the largest surge at 4,000 starting watts. Your peak demand is not 3,500 and it is not 4,000. It is the running total minus the well pump’s running figure, plus the pump’s full starting figure. In practice that often lands a couple thousand watts above your running total, which is why a generator sized only to the steady load will stall the moment the pump cycles on. Always size to the peak.
If the pump is also tied to your heating or cooling, it helps to know those loads first. The furnace size calculator and the energy cost calculator can tell you what those systems pull before you decide what to back up.
Portable vs standby
Two formats cover almost everyone:
- Portable generators usually run from about 3,000 to 8,500 watts. They are cheaper, you fuel them by hand, and you run essential circuits through a transfer switch or heavy-duty cords. Good for the 5,000 to 7,500 watt “essentials” plan.
- Standby (home backup) generators are permanently installed, run on natural gas or propane, and start automatically when the power drops. They commonly range from 10,000 to 26,000 watts, enough to run nearly the whole house, AC included.
Whichever you choose, run any fuel-burning generator outdoors and well away from windows. Carbon monoxide from a generator in or near the house is a serious hazard, and the U.S. Consumer Product Safety Commission publishes safe-placement guidance worth reading before an outage hits.
Get your number
List the appliances you want backed up, total their running watts, add the single biggest starting surge, and add about 10 to 20 percent headroom so the generator is not maxed out. Run your own loads through the generator size calculator to get a sized recommendation with the surge math included. If you only need to cover a fridge and a gas furnace, what size generator for a refrigerator and furnace focuses on those two loads with the surge math worked step by step.