Add up the running watts of everything you’ll run at once, then the starting watts of the biggest motor, such as a central AC. A fridge, lights, a furnace blower and a 3-ton AC come to at least 15.6 kW, or 25.1 kW by Generac’s own AC starting figures if the AC has no soft start kit or load manager.
Fuel
Central AC
Well pump
Sump pump
Furnace blower
Electric water heater
Electric dryer
A fridge and lights are always counted. For anything else, such as an electric range, type the running watts on its label, or a sum such as 2400 + 1500.
Standby generator size
Fridge 800 + lights and Wi-Fi 500 + 3-ton central AC 3,500 + furnace blower 800 = 5,600 W running. Largest start: the 3-ton central AC, 10,000 W. On natural gas, a Generac Guardian 18 kW covers it. Generac rates it 17 kW on natural gas. An AC with no soft start may need more. Generac’s own 3-ton AC figures (about 3.5 to 5 kW running, up to 15 to 18 kW to start) come to 25.1 kW. Generac pairs a 14 or 18 kW unit with a Load Manager or Soft Start kit instead, so ask your dealer which your AC needs.
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How Generac and Champion size a standby generator
Generac adds the running watts of everything that runs at once to the highest single starting watts. Champion adds only the largest motor’s surge above its running watts, then a 20% safety cushion. On Champion’s own example, a fridge, a 1/2 HP sump pump, a 3-ton AC and lights, that’s 14,820 W by Champion’s method and 15,850 W by Generac’s.
The calculator gives the larger of the two. Generac and Champion both say to check each appliance’s nameplate or data plate, and to have a dealer survey the home before you choose a unit.
Running and starting watts for the big loads
| Load | Running watts | Starting watts |
|---|---|---|
| Central AC, 2 ton | 2,500 W | 4,500 W |
| Central AC, 3 ton | 3,500 W | 10,000 W |
| Central AC, 4 ton | 5,000 W | 12,000 W |
| Central AC, 5 ton | 5,500 W | 15,000 W |
| Well pump, 1/2 HP | 1,000 W | 2,100 W |
| Well pump, 1 HP | 2,000 W | 4,000 W |
| Sump pump, 1/3 HP | 800 W | 1,600 W |
| Sump pump, 1/2 HP | 1,050 W | 2,150 W |
| Furnace blower | 800 W | 2,300 W |
| Refrigerator | 800 W | 2,200 W |
| Electric water heater | 4,500 W | No extra |
| Electric dryer | 6,000 W | No extra |
| Central AC, 3 ton, Generac’s figures | 3.5 to 5 kW | Up to 15 to 18 kW |
Champion says an AC’s real draw depends on its SEER rating, its age and whether it has a soft start. Its chart gives water heaters and dryers no extra starting watts.
Standby generator sizes
| Guardian | On propane | On natural gas |
|---|---|---|
| 10 kW | 10 kW | 9 kW |
| 14 kW | 14 kW | 14 kW |
| 18 kW | 18 kW | 17 kW |
| 20 kW | 20 kW | 18 kW |
| 22 kW | 22 kW | 19.5 kW |
| 24 kW | 24 kW | 21 kW |
| 26 kW | 26 kW | 22.5 kW |
Briggs & Stratton says standby generators are mainly rated on propane, and Champion puts the drop on natural gas at around 10% on its own models. Briggs & Stratton’s PowerProtect line comes in 13, 18, 22 and 26 kW.
Champion groups standby units into select circuit essentials at 8.5 to 14 kW, managed whole-home power at 14 to 22 kW, and whole-home power at 22 kW and up.
Square footage is only a starting point
Generac puts most 1,550 sq ft homes at 10 to 18 kW and a 2,000 sq ft house at 20 or 22 kW for whole-home coverage. It also says two neighbors with identical floor plans could need generators 8 kW or more apart, because central air, a well pump and electric appliances change the load.
Load management
A load manager briefly pauses a lower-priority load when the generator nears overload, such as when the central AC and an electric dryer start at the same moment, so a smaller unit can cover more of the home. Generac’s service rated transfer switches manage up to four air conditioner loads, and its Smart Management Modules handle more large loads. Champion offers a load management module too.
Installation and safety
A standby generator is installed by licensed pros. Generac’s manual says only a trained and licensed electrician should do the wiring, and never to connect the generator to a building’s electrical system unless a licensed electrician has installed an approved transfer switch.
Never power a house by plugging a generator into a wall outlet. CPSC calls this backfeeding and warns that it puts utility workers and neighbors on the same transformer at risk of electrocution.
Generac’s manual says a qualified professional must connect the fuel, and its spec sheets say the gas pipe must be sized for the generator at full load. Your local building department may require a permit, and an inspection of the electrical and gas connections before the generator is used.
Carbon monoxide
A standby generator’s exhaust contains carbon monoxide, a colorless, odorless, poisonous gas. CPSC staff recommend at least 25 feet between a standby generator and any opening into the home. After Hurricane Ida, they found exhaust that had traveled at least 25 feet and risen more than 10 feet before getting into homes, most often through attic vents.
NFPA 37 allows 5 feet to an opening, and Generac lists its Guardian units for installation as close as 18 inches to a house, away from doors, windows and fresh air intakes, with air intakes at least 10 ft from the enclosure. CPSC staff found the 5 feet in NFPA 37 too close to keep dangerous levels of CO out. Ask your installer for at least 25 feet from doors, windows and vents, or as far as your site allows.
CPSC says to install CO alarms on every level of the home and outside sleeping areas, battery-operated or with battery backup, meeting UL 2034. For the battery a CO alarm takes, see CO detector batteries.
For a portable generator and its cords, see What Bulb For’s generator extension cord guide.