Generator THD: What It Is and Why It Matters

What THD means on a generator listing

Total harmonic distortion, abbreviated THD, is a word that appears on some generator listings and on maker pages for individual models. It is a specification about the electrical output. A generator produces alternating current that is nominally a sine wave, and THD is the listing term for how far the actual waveform deviates from that ideal shape.

When a listing states a THD figure, it states it as a percentage. A lower stated figure describes an output waveform closer to the ideal sine wave, and a higher stated figure describes one farther from it. The figure is one detail among many, and it is read on the listing and confirmed on the maker's page.

The two words in the term are read separately. Distortion is the general word for a deviation from the ideal shape, and harmonic refers to the additional frequencies that make up that deviation. The listing term combines them into a single figure that states how far the output waveform sits from a pure sine wave.

This article explains THD as a listing word. It does not give measured figures for any model, because this site compares generators on stated details only. For the difference between running watts and starting watts, see the article on running watts versus starting watts.

How to read a THD figure

A THD figure appears in the specification portion of a listing or on the maker's page for the model. It sits alongside running watts, starting watts, fuel type, outlets and the start method, and it is compared the same way: stated figure against stated figure across the models on your shortlist.

The listing and the maker's page may state details differently. The maker's page is the more complete statement of a model's specifications, and it is where you confirm a THD figure that appears on the listing. Maker pages are published by the companies themselves, such as Champion Power Equipment and Generac.

A THD figure applies to the electrical output, not to the fuel, the engine or the runtime. It does not describe how long a generator runs, how much fuel it uses or how loud it is. Those are separate listed details, and each is read from the same listing and maker page.

Which generator types state a THD figure

Inverter-type generators are the category of listings where a THD figure commonly appears. The word inverter describes the generator's method of producing electrical output, and listings that use that word often include a THD figure among the stated details.

Conventional generators, often described in listings with words like open frame or portable, may not state a THD figure at all. That is a difference in what the listing states, not a verdict on the model. Some conventional listings state the figure and many do not.

Inverter-type listings also commonly state other output-related features, such as parallel capability and remote start. The THD figure is one of several output details that appear more often on inverter listings than on conventional listings.

When a THD figure matters to you, the way to shop is to compare listings that state it. A listing that states no THD figure gives you no detail to compare. The distinction between inverter and conventional as listing types is covered in more depth in the article on inverter versus conventional generators.

Why a missing THD figure is not a defect

A listing that does not state a THD figure is not saying the model lacks the specification. It is saying the seller did not state it. Many listings state only a subset of the maker's full specifications, and a missing detail is a reason to open the maker's page, not a reason to discard the model.

The same applies to any specification. Watts figures, fuel type and outlet lists are the details most listings state; a THD figure is a detail that some listings state and others leave to the maker's page. For a full explanation of how to read a generator listing, see the article on generator specifications.

How THD fits into your shortlist

A THD figure is one of the details you can compare when you build a shortlist of generator models. It sits next to stated watts, fuel type, outlets and the start method. The comparison is done on stated details, and the guides on this site sort models by those details.

To use this site, start with the main generator guide and compare the stated details of the models on your shortlist. For models that state a THD figure, compare that figure the same way you compare watts. For models that do not state it, read the maker's page before deciding.

For camping and RV shopping, the camping generator guide and the RV generator guide list models by stated details, and the same reading rules apply.

What this site cannot say about THD

This site compares generators on stated details and does not measure output. Therefore this article cannot tell you which models produce a lower or higher THD figure in practice, and it cannot promise that any stated figure determines how a connected appliance behaves.

Whether a particular device suits a particular generator's output is a question for the maker of the device and the maker of the generator. The reader should follow both manuals and read the specification on the maker's page before buying. Guidance on operating a generator comes from the generator maker's manual and from the public pages published by the CPSC, the CDC and the EPA, which are listed at the end of this article.

The same boundary applies to every specification on this site: the guides compare what listings and maker pages state, and questions about actual measured behavior belong to the makers. For the shopping terms, see the comparison of inverter and conventional generators and the article on generators for sensitive electronics.

What to pick for your situation

Questions

What does THD stand for on a generator listing?

THD stands for total harmonic distortion. It is a listed specification that describes how closely the generator's electrical output resembles a pure sine wave. The figure is stated as a percentage, and you read it on the listing and on the maker's page.

Which generator type states a THD figure on its listing?

Inverter-type generators are the category of listings where a THD figure commonly appears. Conventional or open frame listings often do not state one. When the detail matters to you, compare listings that state the figure.

Is a low THD figure a promise about connected devices?

No. A stated THD figure is a specification, and this site makes no promises about what a generator's output does to a device. Whether a device suits a generator's output is a question for the device maker and the generator maker.

What if the listing does not state THD?

Read the maker's page for the model. A listing states only a subset of the maker's full specifications, and a missing detail is not a defect in the model. The maker's page is the more complete statement of the model's specifications.

Does this site measure THD?

No. This site compares generators on stated details. It does not measure output, runtime, fuel use, noise or any other behavior. The guides compare what listings and maker pages state.