Standby generator sizing is the process of calculating the maximum simultaneous starting and running wattage of your essential electrical loads to select a generator with adequate continuous and surge capacity. Getting this wrong doesn't just mean you run out of power during an outage; it fundamentally changes the physical footprint of your equipment pad, dictates your gas line diameter, and determines whether your automatic transfer switch (ATS) will handle the inrush current without welding its internal contacts shut. The most common mistake DIYers and junior electricians make is confusing running watts (the continuous thermal limit) with starting watts (the momentary magnetic surge required to spin an inductive motor), leading to undersized units that stall and trip on day one.
The Core Theory: Running Watts vs. Locked Rotor Amps (LRA)
To properly execute a standby generator sizing guide, you must understand the physics of inductive loads. Resistive loads (like electric baseboard heaters or incandescent bulbs) draw a linear, predictable current. Inductive loads (motors, compressors, transformers) require a massive spike in current to establish the magnetic field and overcome mechanical inertia. This peak surge is measured on the equipment nameplate as Locked Rotor Amps (LRA).
The Flywheel Effect: Think of an AC compressor motor like a heavy mechanical flywheel. It takes a massive shove to get it spinning from a dead stop, but very little effort to keep it moving once it reaches operating speed. The 'shove' is your starting wattage; the 'keeping it moving' is your running wattage.
When a generator's alternator cannot supply this surge, the voltage dips severely. If the voltage drops below 90V at the compressor terminals, the contactor chatters, the motor fails to transition out of the start winding, and the generator's internal breaker trips to protect the alternator from thermal meltdown. According to the NFPA 110 Standard for Emergency and Standby Power Systems, voltage dip during motor starting must be carefully managed to prevent cascading failures across the rest of the connected electrical system.
Where You Meet This in Practice: Transfer Switches and Fuel Lines
Sizing a generator is not just about the alternator's output; it changes the entire upstream and downstream installation. Here is where the math meets the physical jobsite:
- The Automatic Transfer Switch (ATS): A 200A whole-house switch requires the generator to fault-clear at 10kAIC (kilo-amps interrupting capacity). If you size up to a 48kW liquid-cooled generator, you must verify the ATS can handle the increased available fault current, or you risk an arc flash during a dead short.
- Fuel Line Diameter and Pressure: A standard 22kW air-cooled natural gas generator requires a 1-inch or 1.25-inch gas line delivering at least 7 inches of water column (WC) pressure. Sizing up to a 38kW or 48kW unit often requires upgrading the utility gas meter and running a 2-inch dedicated poly line, adding thousands to the project cost.
- Soft Starters: Modern sizing strategies heavily rely on devices like the Micro-Air EasyStart soft starter. By ramping up the voltage to the compressor over a few hundred milliseconds, these devices reduce the LRA by up to 70%, allowing a 16kW generator to comfortably start a 4-ton AC unit that would otherwise require a 22kW unit.
Worked Numeric Example: Sizing a 200A Residential Service
Let us run the math for a standard 3,500-square-foot home with a 200A main panel, a 5-ton central AC, and a 1 HP well pump. We will apply a practical load schedule rather than a strict NEC Article 220 utility service calculation, as standby sizing prioritizes simultaneous surge management over continuous thermal limits.
| Load Description | Running Watts (Continuous) | Starting Watts (Surge/LRA) |
|---|---|---|
| 5-Ton Central AC (w/ Soft Starter) | 4,500W | 7,000W |
| Electric Water Heater (4500W elements) | 4,500W | 4,500W |
| Well Pump (1 HP, 240V) | 1,200W | 3,600W |
| Refrigerator / Freezer Combo | 800W | 2,400W |
| Lighting, Router, and Receptacles | 3,000W | 3,000W |
| Total Simultaneous Demand | 14,000W (14kW) | 20,500W (20.5kW) |
20.5kW Peak Surge dictates our minimum generator size. A 20kW generator (which typically surges to about 22kW for a fraction of a second) is cutting it too close if the well pump and AC compressor attempt to start simultaneously. The correct specification here is a 22kW to 24kW air-cooled standby unit, such as the Generac Guardian 24kW or Kohler 24RCL, which provides a comfortable buffer for the alternator's magnetic field to stabilize.
Real-World Scenario Walkthrough: The 5-Ton AC Stall
The Setup: A homeowner purchases a 20kW standby generator to cover their entire 200A panel. The home features a 5-ton central AC unit with a factory compressor that has an LRA of 110A (roughly 26,400 starting watts). No soft starter is installed, and the generator is set to manage the whole house via a 200A service-rated ATS.
The Numbers: The 20kW generator provides roughly 83A continuous at 240V, and a momentary surge capacity of about 125A. During a summer grid failure, the house is already drawing 40A in running loads (well pump cycling, refrigerator, lighting, and a window AC unit). Total baseline draw: 40A. The 5-ton AC contactor pulls in, demanding 110A LRA. Total instantaneous surge demand: 150A.
The Outcome: The utility drops, and the generator starts and assumes the load. The thermostat calls for cooling, and the 5-ton AC contactor engages. The generator's alternator cannot supply the 150A magnetic surge. The system voltage sags violently to 160V. The AC compressor stalls, remaining locked in the rotor state, drawing maximum current continuously. The generator's internal 100A main breaker trips on overcurrent, killing power to the entire house and leaving the homeowner in the dark and the heat.
What Went Wrong: The sizing calculation used running watts for the AC and ignored the simultaneous surge overlap. The alternator's magnetic field collapsed under the combined reactive load. This could have been prevented by either upsizing to a 26kW liquid-cooled unit, installing a load-shedding module to drop the window AC before the central AC starts, or installing a compressor soft starter to slash the LRA.
Step-by-Step Sizing Protocol for Inductive Loads
Follow this sequence on the jobsite to ensure your generator specifications match the physical reality of the home's electrical panel.
- Audit the Panel with a Clamp Meter: Do not rely on breaker handle ratings. A 40A breaker feeding an oven might only ever see 18A of continuous draw. Measure the actual baseline load during peak summer evening hours.
- Hunt for Nameplates: Physically locate the data plates on the AC condenser, well pump, and sump pump. Write down the RLA (Running Load Amps) and the LRA (Locked Rotor Amps). If LRA is missing, multiply the RLA by 6 as a conservative baseline for older compressors.
- Apply Diversity and Load Shedding: Not everything starts at the exact same millisecond. Program the ATS load-shedding module to prioritize life-safety and HVAC, delaying the electric water heater and EV charger by 5 to 10 seconds to prevent surge overlap.
- Mandate Soft Starters for Large Tonnage: For any central air system 3 tons or larger, factor a soft starter into the project budget. It reduces the required generator size by one full tier, saving money on the generator, the gas line upgrade, and the concrete pad.
- Match the ATS to the Generator, Not Just the Panel: Ensure the ATS continuous amperage rating matches the generator's maximum continuous output current, and verify the fault-current rating exceeds the available short-circuit current at the generator terminals.
Frequently Asked Questions
Can I just buy a massively oversized 48kW generator to avoid doing the math?
No. While it solves the surge problem, severely underloading a generator causes its own failures. In diesel generators, running at less than 30% capacity causes 'wet stacking' (unburned fuel washing past the piston rings). In natural gas units, the engine governor will hunt and surge, causing frequency fluctuations that can damage sensitive inverter-based appliances and smart home electronics.
How does an existing solar array with battery backup change the sizing guide?
If you have a grid-forming hybrid inverter (like a Sol-Ark 15K or Enphase IQ 5P), the solar and battery system can act as a 'virtual generator' to supply the massive LRA surge for the AC compressor, while the standby generator only needs to be sized for the continuous running watts and battery recharging. This allows you to install a much smaller, highly loaded generator that runs at peak efficiency.
Does altitude affect my standby generator sizing?
Yes. Air-cooled natural gas and propane generators lose approximately 3% to 5% of their rated output for every 1,000 feet of elevation above sea level due to thinner air reducing engine combustion efficiency and alternator cooling. If you are installing a 22kW unit in Denver (5,280 ft), you must derate the unit by roughly 15%, meaning it will only produce about 18.7kW in practice. Always size up one tier in high-altitude installations.
