What Size Generator Do I Need for My House?
The size generator you need for your house is the total running watts of everything you want on at once, plus the single biggest starting surge on top. For whole-house standby power, Generac says a 2,000 sq. ft. home typically needs a 20 kW or 22 kW unit, but your motors (well pump, air conditioner) matter more than your square footage.
How generator sizing works
Every motor pulls a surge when it starts, then settles to its running load. Generac puts that starting figure at often two to three times the running number, and its support pages say motor appliances can hit 3 to 4 times running wattage. Lights and electronics barely surge at all.
That's why generators carry two ratings. Generac's 6500 portable is rated 6,500 running watts and 8,125 starting watts. Its iQ3800 inverter is rated 3,000 running and 3,800 starting. The running number is what it can hold all day. The starting number is the short burst it can give a motor.
Generac's formula is simple: total kW required equals the sum of all running wattages plus the highest single starting wattage. You only add one surge, because you start motors one at a time.
Step by step
- Decide what you're backing up. Generac's tool splits it into Basics Only (lights, refrigerator, medical devices) and Basics Plus, which adds things like the garage door, washer and dryer, and cooking.
- Read each appliance's nameplate or spec sheet for its wattage, the way Generac recommends. Don't guess the well pump; the label is on the pump control box or in the paperwork from the well driller.
- Add the running watts.
- Find the biggest starting surge on the list and add it once.
- Pick a generator whose running rating covers that total, with room to spare.
Do the list with a pencil in the basement, not on your phone in an outage. You'll find a furnace blower or a second fridge you forgot.
The numbers that move the answer
Central air is the big one. Generac says a 3-ton central air conditioner typically needs about 3.5 kW to 5 kW running, and can demand up to 15 kW to 18 kW to start the compressor. That single surge is why homes with central air end up with large standby units.
Generac's other benchmarks by house size:
| Home size | Generac's typical size | Note |
|---|---|---|
| 1,550 sq. ft. | 10 kW to 14 kW | With one or two load managers to prioritize the AC and well pump; a heat pump may call for 18 kW |
| 2,000 sq. ft. | 20 kW or 22 kW | Whole-house coverage |
| 2,500 to 3,000 sq. ft. | 22 kW | Generac calls it the industry standard for whole-home protection |
Generac lists the well pump with central air, an electric range, an EV charger and an electric dryer as its heaviest loads. On a well, the pump's starting surge often decides whether a mid-size portable works. Our well pump generator guide covers that load in detail.
Portable, inverter or standby
Small inverter generators are for a few plug-in loads. Generac notes most portables have only a couple of 120-volt outlets and can run one or two plug-in appliances without changes to the house. Fine for a fridge, a phone charger and a lamp.
To feed hard-wired circuits through a transfer switch, you need a bigger portable. Generac's 30-amp transfer switch for 1 to 2 circuits is built for generators starting at 5 kW, and its whole-house switch with a 50-amp generator side starts at 8 kW. Above that, a standby unit makes more sense than wrestling the biggest portables into the yard every storm. Compare the options in portable generators for home backup and whole house standby generators.
What to do next
Once you have your number, decide how it connects. A portable on an interlock or transfer switch gets the circuits you choose; a standby unit with an automatic switch gets the whole panel. Start with how to connect a generator to your house safely.
FAQ
What size whole house generator do I need?
Generac's rule of thumb is a 20 kW or 22 kW unit for a 2,000 sq. ft. home and 22 kW for houses from 2,500 to 3,000 square feet. Central air, electric heat and a well pump can push you higher. A dealer load calculation beats any square-footage chart.
What size standby generator do I need?
Add your running watts plus the biggest starting surge, same as a portable. The difference is that a standby unit usually covers the whole house, so the air conditioner surge sets the floor. Generac's online finder asks about home size, heating type and EV charging to narrow it down.
What size generator to run a 1,500 sq ft house?
Generac's closest example is a 1,550 sq. ft. home, where a 10 kW to 14 kW unit with one or two load managers can prioritize the AC and well pump. A home with a heat pump or high-draw medical equipment may need 18 kW. Without central air, a smaller unit often works.
What size generator do I need for a 1,200 square foot house?
Generac doesn't publish a figure for that size, and square footage is a weak guide anyway. List your loads, add the running watts and the biggest surge, and size from that total. A small house with a well pump and central air can need more than a big house on city water with gas heat.
What size generator do I need for a 3,000 square foot house?
Generac calls a 22 kW generator the standard for whole-home coverage on houses from 2,500 to 3,000 square feet. Two air conditioners or electric heat can push that up. If you only back up essentials, a much smaller unit will do.
Sources
- GeneracWhat Size Generator Do I Need? (opens in a new tab)
- GeneracWhat Does It Mean When My Generator Displays an Error Code RPM Sense Loss (1501, 1505, 1511, 1515)? (opens in a new tab)
- GeneracPortable Generator, 6500, with COsense (opens in a new tab)
- GeneracPortable Inverter Generator, iQ3800 Dual Fuel (opens in a new tab)
- Generac30-Amp 1-2 Circuit Manual Transfer Switch (opens in a new tab)
- GeneracWhole House 200A Utility / 50A Generator Manual Transfer Switch (opens in a new tab)
- GeneracFind the Right Generator (Solution Finder) (opens in a new tab)
- GeneracPortable vs. Home Standby Generators (opens in a new tab)
- U.S. Consumer Product Safety CommissionPortable Generator Safety Alert (opens in a new tab)