Generator sizing guidelines are the systematic calculations used to match a generator’s continuous and surge power output to the specific running and starting electrical loads of a building. Getting this math right dictates whether your 200-amp automatic transfer switch (ATS) holds steady during a compressor startup, or if the voltage sags below 190V and permanently fries the control board on your modern HVAC system. The most common confusion in this space is mixing up running watts (the steady-state thermal limit) with starting watts (the 2-3 second inrush current required to spin an inductive motor), which leads homeowners to buy undersized 10kW units that instantly trip the main breaker the moment the well pump kicks on.
The Core Math: Running Watts vs. Starting Watts
To size a generator correctly, you must account for both the continuous thermal load and the momentary magnetic inrush current. Think of running watts as highway cruising speed, and starting watts as the massive burst of fuel needed to get a heavy truck moving from a dead stop. Motors (like those in AC compressors, well pumps, and refrigerator fans) require 2 to 3 times their running wattage to overcome initial inertia and establish a magnetic field. This is often listed on the equipment nameplate as LRA (Locked Rotor Amps).
Worked Numeric Example: The 200A Partial-Home Load
Let us size a system for a realistic partial-home load: a 3-ton central AC, a 1 HP well pump, a refrigerator, and basic lighting/Wi-Fi.
- 3-Ton Central AC: Running = 3,500W. Starting (LRA surge) = 10,500W.
- 1 HP Well Pump (240V): Running = 1,500W. Starting = 4,500W.
- Refrigerator: Running = 700W. Starting = 2,100W.
- Lights/Router/Sump Pump: Running = 1,200W. Starting = 1,200W (sump pump surge absorbed in total).
The Calculation:
Total Running Watts = 3,500 + 1,500 + 700 + 1,200 = 6,900W.
Highest Single Starting Surge = 10,500W (the AC compressor).
Minimum Surge Requirement = 6,900W + 10,500W = 17,400W (17.4kW).
We then apply a 20% derating buffer. Running an alternator at 100% capacity causes excessive voltage drop and shortens the engine life. 17.4kW × 1.2 = 20.88kW. This math points directly to a 22kW generator class.
Where You Meet Generator Sizing Guidelines in Practice
In the field, generator sizing is not just about the engine block; it is about the physical interface with your home's electrical infrastructure. You meet these guidelines at the Automatic Transfer Switch (ATS) and the main service panel busbars.
If you have a standard 200A residential service, industry leaders like Generac and Kohler typically pair a 200A ATS with a 22kW or 24kW air-cooled generator. The physical wiring for this setup usually requires 1/0 AWG aluminum or 2 AWG copper THHN conductors routed through 1.5-inch PVC conduit from the generator to the ATS.
A 22kW generator outputs roughly 91.6 Amps per leg (Leg A and Leg B) at 240V. A 200A home panel provides 200A per leg. If your electrician places all your 120V lighting, outlets, and the 120V control circuits of your HVAC on Leg A, that leg will overload and trip the generator's internal 90A breaker—even if your total house wattage is only 15kW. Proper sizing requires the installer to physically balance the 120V branch circuits across both busbars during the ATS installation.
If your calculated load exceeds 24kW, or if you have a 400A Class 320 service, you cross the threshold from air-cooled to liquid-cooled generators. Liquid-cooled units (like the Kohler 38RCL) are vastly more expensive, require plumbing-like coolant maintenance, and consume significantly more fuel, which is why accurate sizing to avoid oversizing is critical.
The Decision Tree: Picking Your Exact kW Rating
Use this decision matrix to terminate your sizing calculations and select a concrete generator class. Do not guess; follow the load profile.
| Total Running Load | Major Inductive Loads | Service Panel Size | Concrete Pick / kW Rating |
|---|---|---|---|
| Under 8 kW | No Central AC, Gas Heat | 100A - 200A | 10kW Air-Cooled (e.g., Generac 7171) |
| 8 kW - 12 kW | One 2-3 Ton AC, Gas Heat | 100A - 200A | 14kW - 16kW Air-Cooled |
| 12 kW - 18 kW | One 4-5 Ton AC OR Well Pump + 3-Ton AC | 200A | 22kW Air-Cooled (e.g., Generac Guardian 7043) |
| Over 18 kW | Multiple ACs, Electric Heat, or 400A Service | 200A - 400A | 26kW+ Liquid-Cooled (e.g., Kohler 38RCL) |
The Default Recommendation: For 90% of standard US homes with a 200A main panel, central air conditioning, and gas appliances (water heater, furnace, stove), the definitive pick is a 22kW air-cooled standby generator. It provides enough surge headroom for a 5-ton AC compressor while staying within the physical ampacity limits of a standard 200A ATS without requiring expensive load-shedding relay modules.
Common Sizing Mistakes That Trip Breakers or Fry Boards
- Sizing for Electric Resistance Heat: Electric baseboards or tankless water heaters draw 10kW to 18kW continuously. Sizing a generator to cover electric heat will push you into the $15,000+ liquid-cooled tier. The fix: Install a 200A ATS with a built-in load-shedding contactor that physically locks out the electric heat breakers during an outage, allowing you to use a much cheaper 22kW unit to run the rest of the house.
- Ignoring Solar Inverter Pass-Through: If you have a hybrid solar setup (like a Sol-Ark 15k or FranklinWH), the inverter itself requires power to run its internal relays and charge the 48V battery bank. A 48V battery charger can pull 4,000W to 6,000W continuously. If you forget to add this to your 'Running Watts' column, the generator will bog down and shut off when the batteries try to equalize.
- Assuming Nameplate Watts are Accurate: Older HVAC units degrade. A 10-year-old 3-ton AC that nameplates at 3,500W might actually draw 4,200W due to worn bearings and dirty coils. Always clamp the actual running amps with a multimeter during the site survey before finalizing the generator size.
FAQ: Edge Cases and Transfer Switch Limits
Q: Can I install a 24kW generator on an older 100A main panel?
Yes, but the ATS must be rated for the smaller of the two: the generator's max output or the panel's main breaker. In this case, you must install a 100A-rated ATS. The generator will simply never be loaded past its 100A (24kW) physical limit, which is perfectly safe and code-compliant.
Q: Do I need to size my generator to handle my Level 2 EV Charger?
No. EV chargers (which pull 7kW to 11kW) are massive, non-essential loads during a grid failure. The standard practice is to physically lock out the EV charger breaker in the subpanel or use a smart ATS relay to drop the charger offline the millisecond the grid drops. Do not waste thousands of dollars upsizing your generator for a car charger.
Q: What happens if the generator is too large for the load?
Unlike portable inverter generators, large standby units do not suffer from 'wet stacking' (unburned fuel in the exhaust) just because the house is only drawing 2kW at 3 AM. Modern standby engines regulate fuel delivery via electronic governors. However, buying a 40kW liquid-cooled unit for a 10kW load is a waste of capital and increases annual maintenance costs (oil, coolant, filters) unnecessarily.
