Generator Sizing for Motor Loads: Starting Watts, Locked Rotor and Load Management
A generator that is 20% larger than the running load can still fail on the well pump. Motor starting is the design case, and load management is how you avoid buying a bigger unit.
An induction motor draws locked-rotor current at start - typically 5-7× full-load current, which translates to 2-3× running watts for several seconds. A generator sized to the running total will stall or trip on the largest motor unless it has the transient capacity to ride through it.
Air-cooled residential generators publish a "surge" rating for exactly this reason. The utility is the surge number against the largest motor, not the sum of nameplates.
| Load | Starting multiple | Sizing implication |
|---|---|---|
| Furnace blower (ECM) | 1.5 - 2.5× | Rarely the binding constraint |
| Well pump (deep, submersible) | 2.5 - 3.5× | Usually the largest single starting load on a rural service |
| Air conditioner / heat pump | 3 - 5× | Soft-start or a hard-start kit materially reduces the multiple |
| Refrigeration compressor | 3 - 5× | Multiple units should be staged, not started together |
| Elevator / large pump (commercial) | 5 - 7× | VFD or across-the-line start decides the generator class |
A load-management module can shed the non-essential loads during generator operation - EV charger, range, dryer, second AC - letting a smaller generator carry the service. On most residential retrofits, load management costs less than the jump from one generator class to the next, and it reduces fuel consumption for the life of the unit.
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1
List every motor with its running and starting watts
Use nameplate LRA where available; otherwise apply the published multiple for the motor type.
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2
Identify the largest starting load
Size the generator surge against this single load plus the other running loads - not against the sum of all starting loads.
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3
Sequence the loads
Staggering AC compressors and pumps reduces the peak the generator must carry; controls can enforce the order.
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4
Add load management before adding kW
Shedding non-essential circuits usually buys more capability per dollar than the next generator size.
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5
Confirm fuel and derate
Natural gas derates the engine; verify the kW rating on the fuel you will actually run.
How much larger should a generator be than the running load?
Do soft starters help?
Can I run a welder or large compressor on a standby generator?
Size a standby system with the design desk
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