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Generator Sizing Estimator for HVAC Equipment

Will a generator run your house and start the air conditioner? Compare its running rating with your load, and its motor-starting capability with the compressor’s locked-rotor kVA.

Method reviewed October 26, 2026. Version 1.0.0. How it works

This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.

Your inputs

Sum of the running watts of everything that can already be on (lights, fridge, furnace blower, other motors), from nameplates or measurements.

For an air conditioner or heat pump: the outdoor unit’s running power (compressor and fan).

Printed on the compressor or equipment nameplate. A soft starter, if fitted, lowers the starting current — use its rated value.

Supply

Starting kVA (or surge rating) from the generator manufacturer’s data, ideally at the voltage dip you can accept.

Result

Looks adequate

Largest motor starting kVA (full voltage)
19.68kVA
Total running load
9,500W

Running 9,500 W (79 %); starting 19.7 kVA vs 20 kVA capability — looks adequate.

  • Generator manufacturers note that motor starting depends on voltage dip, starting power factor and the alternator, and recommend their sizing software for a final selection. Treat this as a first check.
Running load share of generator rating
79%
Starting kVA share of generator capability
98%
Starting kVA if voltage dips 30 %
9.64kVA
Show the working
  1. Starting kVA = LRA × V ÷ 1,000
    82 × 240 ÷ 1,000 = 19.68 kVA
  2. Running load = other loads + largest motor running
    6,000 + 3,500 = 9,500 W (79 % of 12,000 W)
  3. Starting kVA falls with the square of the voltage: at a 30 % dip, 0.7² = 0.49
    19.68 × 0.49 = 9.64 kVA

Running load and starting load

A generator has to carry everything that is already running and then survive the surge when the largest motor starts. For most homes that motor is the air conditioner or heat pump compressor.

Starting kVA at full voltage = locked-rotor amps × volts ÷ 1,000 (× 1.732 for three-phase). Kohler notes that starting current is typically about six times full-load current.

Why this is only a first check

When a motor starts on generator power, the voltage dips, and the starting kVA falls with the square of the voltage: a 30 % dip cuts it to 49 %. How deep a dip your equipment tolerates, the starting power factor and the alternator design all matter, which is why generator manufacturers provide their own sizing software.

Use this estimate to shortlist generator sizes, then confirm with the generator manufacturer. A soft starter on the compressor can lower the starting current substantially.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Running load while the largest motor starts
6,000 W
Running watts of the largest motor
3,500 W
Locked-rotor amps (LRA) of the largest motor
82 A
Motor voltage
240 V
Supply
Single-phase
Generator continuous (running) rating
12,000 W
Generator motor-starting capability
20 kVA

Result

Largest motor starting kVA (full voltage)
19.68 kVA
Total running load
9,500 W
Running load share of generator rating
79 %
Starting kVA share of generator capability
98 %
Starting kVA if voltage dips 30 %
9.64 kVA

Running 9,500 W (79 %); starting 19.7 kVA vs 20 kVA capability — looks adequate.

Assumptions

  • Only the largest motor starts while everything else is already running.
  • Full-voltage starting kVA from the nameplate LRA; generator capabilities are entered from the manufacturer’s data.

Limitations

  • Voltage dip, frequency dip and starting power factor are not modelled; manufacturers’ sizing software should be used for the final choice.
  • A permanently connected generator needs a transfer switch installed by a licensed electrician; never back-feed a home through an outlet.

Frequently asked questions

Where do I find locked-rotor amps?

On the outdoor unit’s nameplate or the compressor label, marked LRA.

Can I plug a generator into an outlet to power the house?

No. Back-feeding is dangerous for you and for utility workers. A licensed electrician must install a transfer switch or interlock.

What generator rating should I compare with starting kVA?

The motor-starting kVA or surge rating from the generator manufacturer’s data, ideally at the voltage dip you can accept.

Sources

  1. Sizing Generators for Motor Starting: A Practical Guide (Power Systems Topics 103) — D. Krueger and R. Van Maaren, Kohler Power Systems. Starting current is typically six times full-load current; starting kVA falls as the square of the voltage dip (a 30 % dip reduces it to 49 %); motor interactions make manual generator sizing difficult, and manufacturers provide sizing software.
  2. University Physics Volume 2, §15.2 Simple AC Circuits — OpenStax, Rice University (CC BY 4.0). Capacitive reactance X_C = 1 ÷ (2π f C), so C = I ÷ (2π f V).
  3. Determining Electric Motor Load and Efficiency — U.S. DOE. Equation 1: three-phase input kW = V × I × PF × √3 ÷ 1,000 (worked example 469.7 V, 37 A, PF 0.763 → 22.9 kW); Equation 2: full-load input kW = hp × 0.7457 ÷ efficiency.

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