Glossary / UPS

THD

THD, or Total Harmonic Distortion, measures how much a UPS waveform differs from an ideal sine wave because of harmonic content. It is usually shown as a percentage for output voltage or input current.

What THD means on a UPS

THD stands for Total Harmonic Distortion. It describes the harmonic content added to an electrical waveform compared with its fundamental frequency, usually expressed as a percentage. A lower percentage generally means less measured distortion under the stated test conditions, but the number is not a complete rating of UPS quality.

On UPSFit product pages, THD may refer to different parts of the system:

Output voltage THD describes distortion in the voltage supplied by the UPS to connected equipment.
Input current THD describes distortion in the current drawn by the UPS from the upstream electrical source.

These are different measurements and should not be compared as though they describe the same behavior. UPSFit listings include output-voltage THD values such as 1%, 2%, 2.5%, 3%, 4%, and 5%, as well as input-current THD values such as 3%, 4%, and 5%. Some listings also identify THD under a linear load or non-linear load.

Why buyers may care

THD can matter when the UPS will support equipment that is sensitive to the quality of its power, or when the installation includes loads that draw current in a non-sinusoidal way. It may also matter when an installation has specific electrical requirements for the UPS output or its interaction with the upstream supply.

The relevant figure depends on the question being asked. Output voltage THD is about the power delivered to the load; input current THD is about the effect of the UPS on the source. A product page that reports one does not necessarily provide the other.

What a THD figure does not tell you

A THD percentage is meaningful only with its measurement context. The label may not state:

Whether the figure applies to the input or output
The load type and load level used for the test
The operating mode or other test conditions
Whether the value is typical, maximum, or measured under a particular condition
Which measurement method or standard was used

A capability flag or a single THD value is not an overall performance or quality rating. Two figures should not be treated as directly comparable unless their measurement conditions are sufficiently similar. Likewise, a product listing that mentions THD does not by itself establish suitability for every connected load.

What to check on a product page

Look for the complete field name and percentage rather than relying on “THD” alone. Confirm whether the specification is for output voltage or input current, and check whether it is stated for a linear load, non-linear load, or another condition. If the page does not provide those details, that part of the product’s behavior remains unknown from the listing.

Nearby specifications such as waveform and power factor describe related aspects of UPS operation, but they are not substitutes for a THD measurement.