A specification is not a compatibility guarantee
Verify the exact generator, transfer equipment, connected electronics, and installation. A published harmonic-distortion value does not replace the equipment maker’s operating instructions or an electrical professional’s assessment.What total harmonic distortion measures
Total harmonic distortion (THD) is one way manufacturers describe harmonic content in an electrical waveform relative to its fundamental component. A THD figure should be read with its test method, load, and applicable model. It does not by itself describe voltage/frequency regulation during a transfer, startup behavior, transient events, every load condition, or compatibility with a particular device.
Use the generator’s exact specification sheet and the electronics manufacturer’s requirements. If a device has strict input-power limits, ask its manufacturer or service provider about operation on the proposed generator/transfer system.
Three exact-model examples—not a brand-wide ranking
| Exact model/document | Published statement | Limit of the statement |
|---|---|---|
| Generac G0073270, 22–28 kW specification sheet | Lists total harmonic distortion as less than 5%. | Applies to the model family and manufacturer document; do not generalize to every Generac unit or every operating condition. |
| KOHLER/Rehlko 26RCA/26RCAL, G4-315 | Lists THD below 5% from no load to full load with a linear load. | The stated load condition is part of the claim; it is not a universal test of every household device or transient event. |
| Champion 201222 | Its product page says the alternator produces less than 5% THD. | This is manufacturer-published product information, not independent testing or a guarantee for a connected device. |
Surge protection addresses a different issue
THD concerns distortion in the AC waveform. A surge-protective device is intended to limit certain transient overvoltages. They are not interchangeable specifications, and a low THD claim does not mean a house is protected from every lightning, utility, switching, or equipment event.
Whether to install a whole-service or point-of-use surge-protective device, and where it belongs in a system with a generator and transfer switch, depends on the electrical design and product listings. Ask the electrician to identify the device, rating, location, and what it does not protect against. Do not open a panel or modify a generator connection yourself.
Questions to ask about the complete system
- What are the exact generator, transfer switch, and surge-device models in the proposal?
- Which published THD, voltage, frequency, and load conditions apply to that generator model?
- Does the transfer sequence create an interruption or voltage variation that matters to the equipment being protected?
- Are any electronic or medical devices subject to specific input-power requirements? Confirm those with the device manufacturer or clinician.
- What inspection, grounding/bonding, and surge-protection work is included and permitted?
When a UPS may be useful
An uninterruptible power supply (UPS) can provide temporary backup and, depending on the model, conditioning for equipment connected to it. Select one by checking its rated output, the connected equipment’s input requirements, and the runtime needed; do not assume any UPS will support every load or generator waveform.
Ask the device maker and a qualified professional to confirm compatibility with the generator, transfer system, and intended load. A UPS does not replace safe generator installation, a correctly rated transfer system, or a CO alarm.