What size generator do you need?
Skip the square-footage cheat sheet. Sizing is a load calculation problem, and load management is why most homes do not need as many kilowatts as a straight NEC math exercise suggests.
The code gives two legal paths
With an automatic transfer switch, NEC 702.4 requires one of two things: size the generator to carry the home's full calculated load under Article 220, or install an energy management system per 750.30 and size the generator to the maximum load that system will allow on at once.
That second path is not a loophole, it is why load-shed modules exist. A straight Article 220 calculation sizes for service-entrance demand and routinely produces an oversized, uneconomical unit. Good installers document which method they used.
Central AC is almost always the driver
The number that matters is locked-rotor amps (LRA) off the outdoor condenser data plate, multiplied by 240 V, not tonnage, not square footage, not RLA.
Well pumps are the second common surprise. A 1 HP submersible can surge 4,000–6,000 watts at start against a running load that looks harmless on paper.
Manufacturers publish motor-starting capacity (kVA at 240 V) for cross-brand comparison, but test conditions differ by brand. Treat kVA tables as directional, not gospel.
Load management in practice
Generac supports up to eight Smart Management Modules per system, each on a unique priority 1–8, shedding loads when frequency drops and restoring them in priority order. Generac's 200A smart ATS can separately manage up to four HVAC loads without added hardware.
Some jurisdictions require load management whenever generator output is less than the panel main breaker rating. Ask whether your quote includes shedding modules or a smart ATS, and which circuits stay protected during an outage.
Common size classes, 2025–2026
| Class | Typical use | Notes |
|---|---|---|
| 10–14 kW | Essential circuits, smaller homes | Often paired with a 16-circuit ATS |
| 18–22 kW | Whole-house with load management | Most common suburban install band |
| 24–28 kW air-cooled | Large homes, multiple HVAC | Generac 28 kW (2025) is largest air-cooled on market |
| 32 kW+ liquid-cooled | Large estate, commercial-adjacent | 2.6× equipment cost step from largest air-cooled |
Existing homes: the underused option
NEC 220.87 lets an electrician size from recorded demand data, a year of maximum demand, or the average of 15-minute peaks over at least 30 days, instead of a paper calculation. On a retrofit, that can justify a smaller unit than a textbook Article 220 exercise.
What to ask on the first call
- Was a load calculation performed? Which NEC method?
- What is the largest motor-starting load (usually AC LRA)?
- Does the quote include load management hardware?
- Whole-house (service-rated ATS) or essential-circuits subpanel?
- Natural gas or propane, and does the kW rating reflect derate on your fuel?
Consumer Reports adds a sizing-economics point worth repeating: smaller units cost almost as much to install as larger ones, so the marginal cost of stepping up in kW is small relative to the total project, but only after the load calc confirms you need the capacity.
Ready to find who to call?
Open your state page or search by ZIP for a distance-sorted list.
Common questions
- What size generator do I need for my house?
- There is no square-footage answer. NEC Article 702 gives two compliant paths with an automatic transfer switch: size for the full Article 220 calculated load, or install a load-management system and size for the maximum load that system will allow on at once. Central AC locked-rotor amps (LRA × 240V) are usually the sizing driver.
- Does a 2,000 sq ft home need a 22 kW generator?
- Not automatically. Square footage enters the NEC optional calculation only as 3 VA per square foot for general lighting, one input among many. Two 2,500 sq ft homes can differ by 15 kW depending on HVAC, well pumps, heat type, and EV chargers.
- What is load management and why does it matter?
- Load management (Generac Smart Management Modules, smart ATS circuits, etc.) sheds non-essential loads when generator frequency drops. That is why a 22 kW unit can back a 200A service, the code explicitly permits sizing to the managed load, not the panel main breaker rating.
- How do I size for central air conditioning?
- Read locked-rotor amps (LRA) off the outdoor condenser data plate and multiply by 240 volts. That starting surge, not running watts or tonnage, determines whether the generator can start the compressor. Soft starters can cut inrush roughly in half.
- Air-cooled or liquid-cooled?
- Most suburban whole-house jobs use air-cooled units (10–28 kW). Liquid-cooled starts around 24–32 kW and costs more than double at the equipment level. The crossover is a budget and noise conversation, not a square-footage rule.