Backup generator sizing is the process of calculating the total continuous and starting wattage of your essential electrical loads to select a generator with adequate kilowatt capacity and surge capability. It is not about matching your main service panel’s amperage; it is about matching the physics of your specific motor-driven appliances. Getting this wrong changes everything in your physical installation: it dictates whether your transfer switch can handle the inrush current, whether your inlet box wiring will overheat, and whether the generator's alternator has enough rotational mass to start a well pump without stalling the engine.
The most common mistake DIYers and even some junior electricians make is confusing running watts (continuous load) with starting watts (surge or Locked Rotor Amps). A second massive error is sizing the generator to the main breaker (e.g., "I have a 200-amp panel, so I need a 48kW whole-house generator"). In reality, during an outage, you are only running essential loads, and managing the sequence of those inductive loads matters far more than raw continuous capacity.
Where You Meet Backup Generator Sizing in Practice
You meet this concept at the intersection of your transfer switch, your inlet box, and the generator’s alternator. When a motor-driven appliance like a central air conditioner or a submersible well pump starts, it draws a massive spike of current to overcome initial inertia. This is known as Locked Rotor Amps (LRA).
If your backup generator sizing ignores LRA, the alternator cannot supply the instantaneous magnetic field required by the motor. The result is severe voltage sag. If the voltage on a 240V circuit sags below 190V, the AC compressor's internal thermal overload will trip to prevent the windings from melting. The compressor will then refuse to restart for hours, leaving you with a running generator but a hot house. Proper sizing ensures the generator's surge capability (often lasting only 10 to 30 seconds) can absorb this LRA spike while maintaining voltage above the 10% sag threshold required by modern appliance control boards.
A Worked Numeric Example: Sizing for a Standard Home
Let’s look at the math for a typical home running a refrigerator, a 1HP well pump, and a 3-ton central air conditioner. We will use real data plate values, not generic estimates.
- Refrigerator: 700W running / 2,100W starting surge.
- Well Pump (1HP, 240V): 1,500W running / 4,500W starting surge.
- 3-Ton AC (RLA 15A, LRA 90A): Running = 15A × 240V = 3,600W. Starting (LRA) = 90A × 240V = 21,600W.
If you simply add the running watts, you get 5,800W. A beginner might buy an 8,000-watt portable generator and think they have plenty of headroom. But generators must be sized for the worst-case simultaneous surge.
We calculate the required surge capacity by taking the total running watts of all essential loads, and adding the starting surge of the largest single motor (minus its own running watts, since it transitions from running to starting).
The Calculation:
Base Running Load: 3,600W (AC) + 1,500W (Pump) + 700W (Fridge) = 5,800W.
Add AC Surge Delta: 21,600W (LRA) - 3,600W (RLA) = 18,000W.
Total Required Surge Capacity: 5,800W + 18,000W = 23,800W (23.8kW).
To run this house comfortably without manual load shedding, you need a 24kW air-cooled standby unit (like a Generac Guardian 24kW) or a 20kW liquid-cooled unit paired with an automatic load-shedding transfer switch that temporarily drops the well pump when the AC kicks on.
Real-World Scenario Walkthrough: The 10kW Portable Mistake
The Setup: A homeowner purchases a 10,000-watt portable generator (12.5kW peak surge) and installs a 50-amp manual transfer switch to backfeed their 200-amp panel. Their goal is to run their 4-ton AC, well pump, and lights during a summer grid failure.
The Numbers: The 4-ton AC has an LRA of 110A (26,400W surge). The well pump needs 4,500W to start. The baseline running load of the house is about 6,000W.
The Outcome: The grid drops. The homeowner fires up the portable generator, flips the transfer switch, and turns on the AC breaker. The AC contactor pulls in with a loud clack. The generator engine bogs down violently, the exhaust shoots black smoke, and the system voltage sags to 160V. The AC compressor hums, fails to spin the rotor, and trips its internal thermal overload. Seconds later, the generator's low-voltage protection shuts the alternator off entirely to save the electronics.
What Went Wrong: The homeowner sized the generator based on the running wattage sticker on the appliances, completely ignoring the LRA on the AC compressor data plate. Furthermore, they attempted to start the heaviest inductive load first. To fix this without buying a massive standby unit, they would need to install a Micro-Air EasyStart on the AC compressor (which reduces LRA by up to 70% by ramping up the compressor speed) and implement manual load shedding—turning off the well pump and water heater breakers before starting the AC.
Step-by-Step: How to Calculate Your Actual Load
- Audit Essential Loads Only: Ignore electric baseboard heat, EV chargers, and electric ovens. Unless you have a $15,000+ budget for a 40kW+ liquid-cooled generator, you will be shedding these loads manually or automatically during an outage.
- Read the Data Plates: Go to your HVAC condenser and well pump controller. Look for RLA (Rated Load Amps) or FLA (Full Load Amps) for running calculations, and LRA (Locked Rotor Amps) for surge calculations. Do not rely on the breaker size; a 40A breaker protecting a 15A RLA compressor is sized for wire protection and startup tolerance, not continuous draw.
- Convert Amps to Watts: Multiply the amps by the system voltage (120V for standard outlets, 240V for heavy appliances). Example: 15A RLA × 240V = 3,600W.
- Apply the Largest Motor Rule: Sum all continuous running watts. Then, identify the single largest motor in your essential load list. Add its LRA starting watts to the total, but subtract its running watts to avoid double-counting.
- Factor in Power Factor (PF): Older well pumps and industrial motors may have a PF of 0.8. If your generator is rated in kVA (kilovolt-amps) rather than kW, divide your required kW by the PF to find the minimum kVA rating required from the alternator.
Frequently Asked Questions
Q: Can I just use a 200A automatic transfer switch with a 20kW generator?
A: Yes, but the transfer switch must have a built-in load management module (like the Generac Smart Management Modules). A 20kW generator can only output about 83 continuous amps at 240V. If the utility power returns and the 200A ATS reconnects the house while the AC, dryer, and oven are all calling for power simultaneously, the 20kW generator will instantly overload and trip its main breaker before the utility contactor can fully close.
Q: Do inverter generators handle motor surges better than open-frame portables?
A: No. Inverter generators (like the Honda EU7000is) are excellent for clean, low-THD power for sensitive electronics, but their surge capacity is strictly limited by the DC bus capacitors and the inverter module's silicon. An open-frame portable with a massive copper-wound alternator (like a Generac GP15000) can often sustain a 2-second LRA spike much better than an equivalently sized inverter unit, simply due to the physical flywheel mass and magnetic inertia of the larger alternator.
Q: What is the exact NEC requirement for optional standby sizing?
A: NEC Article 702 states that optional standby systems must have adequate capacity to handle the loads connected to them. Unlike legally required standby systems (Article 701) which require strict 2-hour battery backups and rigid load calculations, Article 702 allows the user and installer to select which loads are backed up, provided the equipment is rated for the calculated load and the installation prevents inadvertent backfeed to the utility grid.
