Generator sizing questions almost always start with square footage. That's a reasonable starting point — but it's not the number that actually determines the right size.
Square footage is a proxy, not the real answer
Home size correlates roughly with how many essential circuits a house has — but two homes of the same size can need very different generator capacity depending on whether one has a well pump, central air, or a second refrigerator and the other doesn't. We've built starting-point estimates for common home sizes, from a 1,000 sq ft home up to a 5,000+ sq ft home, but treat these as a starting point to adjust, not a final answer.
Surge watts, not running watts, usually sets the minimum
Motor-driven appliances — well pumps, air conditioners, refrigerators — briefly draw 2-5× their running wattage at startup. A generator sized only around running watts can fail to start these appliances entirely, even though it would run them fine once going. Check every motor-driven load's actual surge requirement with the surge watts calculator before finalizing a size.
Build your actual appliance list
The most accurate approach skips square footage entirely and starts from your specific essential circuits: refrigerator, some lighting, a sump or well pump if present, furnace blower, and — if you want it — central air. Add each one's running and surge wattage in the full generator backup calculator for a precise total rather than an estimate based on home size alone.
Then size fuel to match
Once you know the generator size, fuel logistics are the next question — how many gallons of gas or propane you'll need for a multi-day outage. See the fuel consumption calculator for gas or diesel, or the propane tank size calculator if you're going that route, since propane's lower energy density typically means a larger tank for the same runtime.
Frequently asked questions
Can I undersize a generator and just run fewer things at once?
Yes, this is a common practical approach — a smaller generator with a manual transfer switch lets you choose which circuits to power at any given time, avoiding the cost of sizing for everything running simultaneously.
Do whole-home standby generators need the same surge calculation?
Yes — even a larger standby generator has a surge capacity limit, though it's usually generous enough that surge concerns are less likely to be the binding constraint compared to a smaller portable unit.