Total harmonic distortion describes one aspect of the generator’s electrical waveform. A low THD specification can be useful when comparing equipment, but it does not promise uninterrupted power or guarantee compatibility with every computer, control board, or UPS.
When a proposal says “clean power,” ask what is being specified, under which load conditions, and what the connected equipment requires. A measurable specification is more useful than a reassuring label.
What a THD percentage means
In a 60Hz system, harmonics are components at integer multiples of the fundamental frequency: 120Hz, 180Hz, and so on. Voltage THD compares the combined harmonic voltage with the fundamental voltage. Fluke’s power-quality explanation describes harmonics and their measurement.
Using the common fundamental-referenced definition, THD = combined RMS harmonic voltage ÷ RMS fundamental voltage × 100. “Combined” means the root-sum-square of the harmonic components, not their simple arithmetic sum.
For an illustrative measurement with a 120V fundamental and 3.6V combined harmonic content, THD is 3%. That is not the same as voltage being 3% too high, frequency being 3% off, or losing power for 3% of the time.
Read a real specification in context
Generac’s iQ3800 DF specification lists THD below 3% for the G0089440/G0089460 models. The same document identifies a 120V output and 3,000 running watts on gasoline versus 2,700 on propane. See specification A0006743920 Rev. A, December 13, 2024.
That is a documented portable-generator example, not a recommendation to power an entire business with one. Its low THD does not add 240V output, three-phase capability, or capacity beyond its fuel-specific running rating.
What THD does not tell you
| Question | What to evaluate |
|---|---|
| Will equipment restart during transfer? | Interruption duration and any compatible UPS or other ride-through provision. |
| Will voltage dip when a motor starts? | The generator’s response to the actual load step and the equipment’s tolerance. |
| Will the UPS accept generator input? | Its permitted input characteristics and interaction with the complete load. |
| Will the supply remain within capacity? | Running and starting demand, voltage, phase, and the selected fuel rating. |
Voltage distortion and current distortion are also different measurements. Electronic loads can draw non-sinusoidal current, affecting the supply waveform. Ask a qualified technician to measure the relevant quantities under representative conditions if a compatibility problem needs diagnosis.
Why a UPS can keep using its battery
Schneider Electric’s APC Back-UPS generator guidance describes cases where distorted generator output causes a UPS to remain on battery or switch repeatedly. It also states that no single oversizing factor guarantees compatibility across all systems.
Record the UPS model, load, alarm history, and what else was starting when the problem occurred. Have the equipment suppliers review the combination. Change sensitivity or input settings only through the exact product’s documented procedure and with an understanding of the effect on the protected equipment.
Make compatibility part of the acceptance test
Identify the devices that must keep working and the events they must tolerate: initial outage, generator transfer, large-load starts, and return to utility. Request a test plan that observes those devices under the agreed conditions. A successful no-load generator exercise does not demonstrate that behavior.
See the business sizing guide for the load schedule and transfer-switch planning guide for interruption and transition decisions.
Connected guidance
Technical Topics Related to Clean Power and Total Harmonic Distortion in Generators
Continue exploring this Topic with Single-Phase vs. Three-Phase Generators. Then connect this decision to Is a Whole-Home Generator Worth It? and Sizing a Generator for an All-Electric Home; those adjacent choices can change equipment, installation scope, operating readiness, and total ownership cost.