The first big house I wired for backup power belonged to a couple outside Cleveland. Five bedrooms, two furnaces, a well pump, a wine fridge the husband cared about more than I’d like to admit. They’d already bought a 48 kW generator before I showed up, because a salesman told them a house that size “needs” it. That unit was almost double what they actually used on the worst night of the year.
Square footage is a lousy way to size a generator. What matters is the electrical load you want running at the same time, not how many rooms you have to walk through to reach the kitchen.
So let’s work through how I’d size one, using real wattage figures and a couple of jobs I’ve done. By the end you’ll know roughly what you need and where the money hides.
Key takeaways
- The right generator size for a large house comes from your running load, not your square footage.
- Most large homes land somewhere between 18 kW and 26 kW for whole-house coverage.
- Air conditioning and well pumps have big startup surges that catch people off guard.
- Oversizing costs more up front, wastes fuel, and can even shorten the engine’s life.
Why generator size for a large house isn’t about square footage
A 3,500-square-foot house with gas heat, gas water heating, and one AC unit can pull less power than a 2,000-square-foot all-electric house. I’ve measured it. The all-electric place had an electric range, a heat pump, and an electric water heater all fighting for amps at dinnertime.
What you’re sizing for is the peak moment. That’s usually a summer evening when the AC kicks on while someone’s cooking and the fridge compressor cycles. Or a January morning when the furnace blower, the well pump, and the coffee maker all run at once.
Everything comes down to two numbers per appliance: running watts and starting watts. Motors, compressors, and pumps draw a surge for a second or two when they start, sometimes three times their running figure. If you ignore that surge, you buy a generator that stalls the first time the AC fires up.
Adding up what a big home actually pulls
The honest way to size is to list your loads and add them. I keep a running tally on a clipboard when I walk a house. It’s tedious and it’s the only method that doesn’t lie to you.
Typical running watts for the heavy hitters
| Appliance | Running watts | Starting watts |
|---|---|---|
| Central AC (4-ton) | 4,000 | 12,000 |
| Electric furnace blower | 800 | 2,300 |
| Well pump (1 HP) | 1,000 | 3,000 |
| Electric water heater | 4,500 | 4,500 |
| Refrigerator | 700 | 2,200 |
| Sump pump | 800 | 1,300 |
| Lights, outlets, electronics | 2,000 | 2,000 |
Add the running watts of everything you truly want on at once, then add the single largest starting surge on top. That surge number only counts once, because two big motors almost never start in the exact same instant.
For a full walk-through of this math with more appliances, I’d point you to our step-by-step guide to sizing a generator for your home. And if you want typical figures for smaller stuff like a microwave or a garage door opener, the appliance wattage chart saves you from guessing.
Rent a clamp meter for twenty bucks, or borrow one, and clip it on your main feeders on a hot evening with everything running. A real reading beats a spreadsheet estimate every time.
Whole-house coverage versus picking your essentials
There are two ways to go, and they lead to very different price tags.
Whole-house means the generator carries everything, and you never think about it. For most large homes that’s a standby unit in the 18 kW to 26 kW range running on natural gas or propane. The couple in Cleveland actually needed about 22 kW once we did the tally honestly.
The other route is a managed essentials setup. You cover the furnace, fridge, well pump, some lights, and a few outlets, and you skip the second AC and the electric range. That can drop a large home into a 14 kW to 18 kW unit, which saves real money on both the generator and the gas line.
Load management is the quiet trick
Modern transfer switches can shed loads automatically. When the AC starts, the switch briefly drops the water heater so nothing overloads. This lets a smaller generator behave like a bigger one, and it’s how I’ve fit 20 kW units onto homes that looked like 30 kW candidates on paper. The wiring for this lives in the transfer switch, and if that box is a mystery to you, this breakdown of how transfer switches work is worth ten minutes.
The oversizing trap
Bigger feels safer. It usually isn’t.
A generator that’s twice the size you need still costs more up front, often several thousand dollars, and it burns more fuel at idle. Worse, an engine that rarely sees more than a quarter of its rated load can suffer from wet stacking, where unburned fuel gums up the exhaust over time. I’ve been called out to clean up exactly that.
Never run any generator inside a garage, basement, or breezeway, even with the door open. Carbon monoxide from a standby unit can be fatal. Standby generators belong outside, at least five feet from windows and doors, per manufacturer and code requirements.
The Cleveland couple’s 48 kW unit would have loafed at maybe 20 percent load for years. We swapped it for a right-sized 22 kW, and they got a refund on the difference that nearly paid for the gas line. If you want the full argument with numbers, we lay it out in why oversizing a generator is a costly mistake.
A quick sizing checklist before you buy
Run through this before you sign anything. It’s the same list I mentally check on a site visit.
- List every appliance you want running during an outage, with running and starting watts.
- Add the running watts, then add your single largest starting surge.
- Add about 20 percent headroom, not 100 percent.
- Decide whether you want whole-house or managed essentials coverage.
- Check whether load management could let you drop a size.
- Confirm your gas meter or propane tank can actually feed the unit you want.
That last point trips people up. A 24 kW generator needs a lot of natural gas, and a standard residential meter sometimes can’t deliver it without an upgrade from the utility. Sort that out before the generator lands on your driveway.
If you’re still fuzzy on how much power your particular house draws day to day, spend a few minutes with this look at how many watts a house actually uses. For the big picture on permanent units, the complete guide to whole-house standby generators covers fuel, siting, and maintenance.
Where to point yourself next
Start with the tally, not the showroom. Walk your house on the worst weather day you can imagine, write down what you’d want running, and add the watts. Nine times out of ten, the number is smaller than the salesman’s.
Once you have that figure, get two or three quotes from installers who ask about your loads instead of your floor plan. The good ones will pull out a clamp meter. For the safety and installation standards behind all of this, the Consumer Product Safety Commission’s generator guidance and the wattage planning tips from the U.S. Department of Energy are both worth a read before you commit.
