Step 1: Decide what you're backing up
Essential-circuit (select-circuit) backup
The installer moves a short list of circuits, such as the refrigerator, some lights and outlets, the furnace fan, a well or sump pump, internet, and maybe one AC, to a transfer switch that feeds only those circuits. It costs less and allows a smaller generator, but anything not on the list stays dark. Generac sells smaller units packaged with a 16-circuit transfer switch (Generac).
Whole-home backup
A service-rated transfer switch, typically 200 amps, sits between the utility meter and your main panel, so every circuit can run on the generator. Generac packages its 14–26 kW Guardian units with a 200-amp service-rated “Smart Switch” (Generac brochure). Kohler pairs its generators with its RXT transfer switch (Kohler 20RCA spec sheet). Whole-home doesn't mean every appliance runs at full blast at the same moment. That's what load management (Step 4) is for.
The rule that ties it together
When a standby system transfers automatically, which is the whole point of a standby generator, the electrical code requires that it either supply the full load connected to it, or use an automatic energy management system that keeps the load within the generator's capacity. The load is calculated under NEC Article 220 or another approved method. North Carolina's Office of State Fire Marshal has confirmed that manual load-shedding doesn't satisfy this for automatic systems (NC OSFM interpretation, Article 702). That's why the electrician's load calculation, not a sales sheet, decides the size.
Step 2: Read the rating for your fuel and your climate
- Natural gas vs. propane. Air-cooled home generators are rated lower on natural gas. Generac's 22 kW model is 22 kW on propane and 19.5 kW on natural gas, and its 26 kW model is 22.5 kW on natural gas (Generac brochure). Kohler's 20RCA is 20 kW on propane and 18 kW on natural gas, and its 26RCA is 26 and 24 kW (Kohler 20RCA, Kohler 26RCA). Most Charlotte homes with gas service will use natural gas, so compare those ratings.
- Heat. Generators make less power when it's hot. Kohler's spec sheet gives a general derating guideline of 2% per 10°F above 60°F (Kohler 20RCA spec sheet). Example only: on a 90°F afternoon that guideline works out to about 6% less, so an 18 kW natural gas rating behaves more like 16.9 kW. Generac publishes its own derating figures in its spec sheets. Ask your installer to account for summer conditions.
Step 3: Account for motors that start hard (AC and heat pumps)
Appliances list running watts, but motors briefly draw much more when they start. HomeGuide notes that startup wattage can be about double the running figure for some appliances (HomeGuide). For AC compressors, Micro-Air, a soft-starter maker, puts the surge at 3–5× running watts for 1–3 seconds (Micro-Air).
Where to find your numbers: the outdoor unit's nameplate lists RLA (running amps) and LRA (locked-rotor amps, the worst-case starting current). As a rough upper bound, starting demand in kVA is about LRA × 240 volts ÷ 1,000. Hypothetical example: an LRA of 100 A works out to about 24 kVA for that instant. Generator makers publish a motor-starting figure to compare against. Kohler lists “peak motor starting kVA” on its product pages, for example 41 kVA for the 20RCA and 33 kVA for the 14RCA (Kohler 20RCA, Kohler 14RCA). Ask your installer to do this comparison for your actual equipment.
Heat pumps and electric heat strips
If your home has a heat pump, check the indoor air handler for electric backup heat, often called heat strips or auxiliary heat. It's rated in kW on the label. When it comes on, a 10 kW strip draws about 10,000 watts, more than many whole houses use on a mild day. That matters most in a winter ice storm, exactly when you'd be leaning on the generator. Homes with gas furnaces don't carry this load.
Soft-start kits
A soft starter, installed at the outdoor unit, ramps the compressor up gradually instead of all at once. Micro-Air says its unit cuts startup current by up to 75%, and gives an example of a 2-ton AC whose startup surge falls from about 8,500 to 3,600 watts (Micro-Air, manufacturer claim). A hard-start kit is a different device that helps a weak compressor start but doesn't reduce the surge (same source). Have your HVAC contractor confirm compatibility and any effect on your equipment warranty.
Step 4: Use load management so a smaller unit can do more
Load management, called an energy management system in code terms, keeps big loads from piling on the generator at once. It's often the difference between a generator size that's affordable and one that isn't.
- Generac: its 200-amp Smart Switch “intelligently manages up to four air conditioner loads with no additional hardware,” and up to eight more 240-volt loads with Smart Management Modules (SMMs) (Generac 20/22/24 kW spec sheet). An SMM watches generator frequency, drops its load when the generator is overloaded, and waits five minutes before trying again, with extra delay for lower-priority loads (Generac SMM manual).
- Kohler: its load shed kit automatically manages up to six residential loads, including two independent air conditioners. It requires a Kohler RDC2 or DC2 controller and an RXT or RDT transfer switch (Kohler accessories spec sheet).
Typical candidates to manage are a second AC, an electric dryer, an electric water heater, a pool pump, heat strips, and an EV charger. In practice, managed loads may wait several minutes or stay off during a heavy-load outage. Decide with your installer which loads you can live without for a while.
Worked examples (illustrations, not load calculations)
The running-watt figures below come from HomeGuide's appliance chart (HomeGuide), with an electric water heater at the top of Angi's 3,000–4,500 W range (Angi). Your appliances will differ, and an electrician's Article 220 calculation applies demand factors, so the official number for your home could be higher or lower.
Example A: Essentials, gas furnace
| Load (running watts) | Watts |
|---|---|
| Refrigerator/freezer | 1,200 |
| Well or sump pump | 1,000 |
| Furnace fan | 500 |
| Four lighting circuits | 400 |
| Microwave | 1,000 |
| Television | 300 |
| Desktop computer / home office | 200 |
| Total if all run at once | 4,600 |
If the refrigerator and pump started at the same moment at double their running watts, demand would briefly touch about 6,800 W. A select-circuit system on a small air-cooled unit has comfortable margin here, even after the natural gas derate. The Generac 10 kW model is rated 9 kW on natural gas (Generac brochure).
Example B: Whole home in summer, 4-ton central AC, electric laundry and water heater
| Load (running watts) | Watts |
|---|---|
| Everything in Example A | 4,600 |
| 4-ton central air conditioner | 8,500 |
| Washing machine | 1,200 |
| Electric oven | 2,000 |
| Electric clothes dryer | 3,000 |
| Electric water heater | 4,500 |
| Total if all run at once | 23,800 |
That's more than a 22–26 kW air-cooled unit delivers on natural gas, and it doesn't count the AC's starting surge. With load management shedding the dryer and water heater while the AC runs, the managed peak in this example drops to about 16,300 W plus the AC start. That's within reach of a mid-20s kW unit, especially with a soft starter on the AC. This is the scenario where load management usually pays for itself.
Example C: Winter ice storm, heat pump with 10 kW heat strips
| Load (running watts) | Watts |
|---|---|
| Everything in Example A (minus furnace fan, plus air-handler fan; treated as equal here) | 4,600 |
| Heat pump compressor (assumed similar to the 4-ton AC above) | 8,500 |
| 10 kW electric heat strips | 10,000 |
| Total before cooking, laundry, or hot water | 23,100 |
For an all-electric home, the winter case can drive sizing more than the summer case does. Options include managing or staging the heat strips, accepting a cooler house during an outage, or choosing a larger unit. Make sure your installer sizes for both seasons.
Bring this to your quote appointments
- AC and heat pump nameplate photos (tonnage, RLA, LRA) and heat-strip kW rating
- A list of gas appliances, which also matters for the gas meter check
- Main panel size (amps) and any subpanels
- Well pump, sump pump, pool or spa, EV charger, medical equipment
- Which loads you could live without for a few hours
- A year of Duke Energy bills, if available, since some electricians use past usage data
Then ask each installer to show you the load calculation, the rating they used for your fuel, and which loads (if any) will be managed. Brand matters less than fit. Our Generac vs. Kohler comparison covers how the two biggest brands handle load management differently.
Not sure where you land? Ask for a sizing visit and free quotes. Choose “Not sure / sizing help” on the form and we'll pass that along to independent local installers.
Sources
- NC Office of State Fire Marshal: 2023 Electrical Code interpretation, Articles 702/705/706/710 (full load or EMS requirement)
- Generac: 10–26 kW Guardian Series brochure (LP/NG ratings, transfer switch packages)
- Generac: 20/22/24 kW Guardian spec sheet (Smart Switch load management)
- Generac: 50 A Smart Management Module manual
- Generac: Home Standby Generators (16-circuit switch packages)
- Kohler 14RCA, Kohler 20RCA, Kohler 26RCA product pages (ratings, motor-starting kVA)
- Kohler 20RCA spec sheet G4-272 (derating guideline, RXT switch)
- Kohler residential accessories spec sheet (load shed kit)
- Micro-Air: What size generator do I need to run my central AC? (manufacturer claims on soft starters)
- HomeGuide: Whole House Generator Cost (appliance running watts, size chart)
- Angi: Whole-House Generator Cost (square-footage guidance, water heater wattage)
- Mecklenburg County Code Enforcement: Generator Safety
Last reviewed October 14, 2026. This page is general information, not an electrical design or load calculation. A North Carolina–licensed electrician must size and permit your system.
Frequently asked questions
What size generator do I need for a 2,500 square foot house?
Square footage alone can't answer it, and published rules of thumb disagree. Angi suggests 12–16 kW for 2,500 sq ft (Angi), while HomeGuide puts a full 1,500–3,000 sq ft home at 17–20 kW (HomeGuide). Your air conditioning, electric heat, and electric appliances matter far more than floor area. A licensed electrician's load calculation is what decides it.
Will a 22 kW generator run my whole house?
It depends on your loads and on load management. Note that a “22 kW” air-cooled unit is rated lower on natural gas: Generac rates its 22 kW model at 19.5 kW on natural gas (Generac brochure). Many homes can run most circuits on a unit this size when the transfer switch sheds big loads like a second AC or an electric dryer. A home with electric heat strips or several large appliances may need more.
What is the difference between whole-home and essential-circuit backup?
Whole-home backup uses a service-rated transfer switch so every circuit in the house can run on the generator, usually with load management to keep the total within the generator's capacity. Essential-circuit backup moves only selected circuits (refrigerator, lights, furnace fan, well or sump pump, internet) to a separate panel, which allows a smaller generator.
Do I need a soft starter for my air conditioner?
Not always, but it can help. A central AC compressor draws a brief surge when it starts that is several times its running load. Micro-Air, which makes soft starters, says its product cuts startup current by up to 75% (Micro-Air). That can let a generator start the AC without an overload. Ask your HVAC contractor whether a soft starter is compatible with your system and whether it affects the equipment warranty.
Does natural gas reduce a generator's output?
Yes, for typical air-cooled home units. Kohler's 20 kW model is rated 20 kW on propane and 18 kW on natural gas (Kohler 20RCA), and Generac's 22 kW is rated 19.5 kW on natural gas (Generac brochure). Size against the rating for the fuel you will actually use.
Who has to do the load calculation?
A licensed electrician. The electrical code requires an automatically transferred standby system to supply the full connected load, or to use an automatic energy management system, calculated under NEC Article 220 or another approved method (NC Office of State Fire Marshal interpretation). The install also needs a Mecklenburg County permit and inspection (Code Enforcement). Online calculators and the examples on this page are only for getting oriented.