Generator Sizing for Motor Surge & LRA
Determine whether a portable or onboard generator has enough peak surge capability to start inductive A/C compressors and water pumps under real-world altitude deratings.
Enter the values above and click Calculate to view recommendations and engineering specifications.
Why Air Conditioners Stall Small Generators
Calculation Assumptions
- Inductive compressor locked-rotor amps (LRA) create a 5×–6× starting surge over running amps.
- Combustion engines derate ~3.5% per 1,000 feet elevation and ~1% per 10°F above 77°F.
Engineering Limitations
- Inverter generators provide electronic surge limits, whereas standard synchronous generators rely on mechanical rotor inertia.
A 15k BTU rooftop unit with 58A LRA needs ~6,900 surge watts. At 5,000 ft, a 2,200W generator derates to ~1,815W and will stall on startup without a soft starter installed.
When an induction motor compressor is stationary and voltage is first applied, it has zero back-electromotive force (back-EMF). Until the rotor spins up to speed (approximately 100 to 300 milliseconds), the compressor acts virtually as a direct short circuit.
For a standard 13,500 BTU air conditioner with an LRA of 58A at 120V, the instantaneous power surge is over 5,700 Watts. Even though running power is only 1,500W, a 2,200W generator will instantly stall or drop into inverter overload protect.
How to Solve the Inrush Problem
- Install an Electronic Soft Starter: A soft starter ramps voltage and phase angle to smooth the start, reducing LRA by 65% to 70% (e.g. from 58A down to ~18A / 1,800W), allowing startup on a 2,000W generator.
- Account for Elevation Loss: For boondocking in mountainous areas (e.g. 5,000 to 8,000 ft), a gasoline generator produces 17% to 28% less power than its sea-level rating. Always oversize or derate your calculations.
Frequently Asked Questions
How many watts does an electric motor draw when starting up?
Inductive AC electric motors (air conditioners, well pumps, air compressors) draw between 4 to 7 times their continuous running current during the initial 100 to 500 milliseconds of starting. This inrush current is defined as Locked Rotor Amps (LRA) on the motor nameplate.
What size generator do I need to start a 1 HP or 2 HP well pump?
A 1 HP deep well submersible pump runs at approximately 1,500W to 1,800W, but starting LRA can reach 35A to 45A at 240V (8,000W to 10,000W surge). Sizing a generator requires at least a 6,500W to 8,000W surge capacity to prevent generator engine stalling and voltage collapse during startup.
What is Locked Rotor Amps (LRA) on a motor nameplate?
LRA (Locked Rotor Amperes) is the maximum instantaneous current the motor draws when full line voltage is applied to the stator windings while the rotor is completely stationary at zero RPM. Once the rotor accelerates to synchronous speed, current rapidly drops down to Full Load Amps (FLA).
How do NEMA motor code letters indicate starting kVA?
NEMA classifies electric motors with code letters (A through V) defining locked rotor kVA per horsepower. Common domestic motors are Code G (5.6 to 6.29 kVA/HP) or Code H (6.3 to 7.09 kVA/HP). Multiplying the motor's rated HP by the code factor determines peak starting kilovolt-amperes.