Anything with an induction motor draws far more current when it starts than when it runs, and for a generator that instant is the design condition.
Why motors surge
At the instant of starting, the rotor is stationary and the motor behaves electrically much like a short circuit. The current is limited only by the winding resistance until the rotor begins to turn and generates a back EMF.
The result is locked rotor amps, several times the running current, lasting a fraction of a second to a couple of seconds depending on the load.
Typical multipliers
| Load | Running | Starting |
|---|---|---|
| Refrigerator | 150–800 W | 2–3× |
| Well pump, 1/2 hp | 900–1,200 W | 2–3× |
| Sump pump | 800–1,050 W | 2–3× |
| AC compressor | 3,500–4,000 W | 2–4× |
| Furnace blower | 500–900 W | 2× |
| Air compressor | varies | 3–4× |
The nameplate on the equipment gives the LRA — locked rotor amps — which is the authoritative figure for that specific machine.
How the sizing works
Sum the running watts of everything that will be on together, then add the starting surge of the single largest motor.
Only one surge is counted, because two motors starting in the same instant is a coincidence rather than a design condition — and where two large motors could plausibly start together, that is what staggering or shedding is for.
What happens if the surge is too large
The generator's voltage and frequency dip. Depending on the severity:
The motor fails to start, drawing locked rotor current until its thermal protection opens. Repeated attempts damage the motor.
The generator's controller shuts down on undervoltage or overload, which is the safe outcome and which leaves the house dark.
Sensitive electronics see a voltage sag, which is what the "clean power" argument for inverter generators is about.
Soft starters
A soft start device limits the inrush to an air conditioning compressor by ramping the voltage or by other means.
The reduction is substantial, and the consequence is that a generator one or two sizes smaller can start the same compressor. Since a generator size step costs several times what a soft starter does, this is the best value component in most installations.
It also reduces the light flicker in the house when the compressor starts, on utility power as well as on the generator.
The electronics side
Modern equipment is not all induction motors.
Variable-speed and inverter equipment — inverter heat pumps, ECM blowers, variable-speed pool pumps — starts gently by design and has very little inrush. A house with inverter HVAC is much easier to back up than one with single-stage equipment.
Electronic loads — computers, televisions, LED lighting — have negligible surge.
That means a household replacing single-stage equipment with inverter equipment is also, incidentally, reducing the generator it needs.
Getting the numbers
Read the nameplates. Running watts or amps, and LRA where given.
Where a nameplate gives amps, multiply by the voltage — 120 or 240 — for watts. Where it gives only running amps and no LRA, assume a multiplier at the top of the range above for sizing, and confirm with the manufacturer if the number is close to a generator size boundary.
Two-pole and three-pole loads
Almost every large motor in a house is 240 V, drawn across both legs of the service.
That matters for a generator because its output has two legs too, and a badly balanced set of loads leaves one leg near its limit while the other is idle.
Whole-house transfer through a panel generally balances itself, since the panel alternates legs. An essential-circuit subpanel assembled without attention can end up with most of the 120 V load on one leg, and it is worth asking whether the installer balanced it.
Generator kW and amps
A generator's rating in kW converts to amps at 240 V by dividing by 240.
A 22 kW unit is roughly 92 amps at 240 V, which is why a 200 amp service on a 22 kW generator needs load management rather than being simply connected — the service can carry more than twice what the generator produces.