What is THD on a generator, and does it matter?

Spec sheets for inverter generators often say "less than 3% THD" or "clean power." Here's what that means in plain words, and how to tell whether your things care.

THD in plain words

The power from a wall outlet is AC. Its voltage swings smoothly up and down, 60 times a second, in a shape called a sine wave.

A generator tries to make that same smooth wave. Some make it very well. Some make a wave with lumps and flat spots in it. THD (total harmonic distortion) is a percentage that says how far the wave is from a perfect, smooth one.

• Lower THD = cleaner power.
• Higher THD = rougher power.

Champion's FAQ puts it simply: THD is "a measure of the deviation of a sine wave from a 'pure' sine wave." It adds that all electricity, even utility power, has some.

What makers list

Generator type	THD makers list	Example
Inverter generator	3% or less	Champion says inverters give "3% or less." Westinghouse lists "less than 3%" for the iGen5000DFc.
Conventional (open-frame) generator	About 12–20%, depending on load	From Champion's FAQ for standard portable generators

Not every maker gives a number. WEN's inverter pages say the power is "clean" and safe for phones, tablets and laptops, but don't list a THD figure. If THD matters for your setup, look for the number in the manual or spec sheet.

Why are inverters cleaner? They convert their power to DC, then build a fresh AC wave electronically. Our inverter vs conventional generator guide explains how that works, along with noise, fuel use and 240 V.

Which devices care?

Most things don't. Champion says the 12–20% from a standard generator is "perfectly acceptable" for common household items like TVs, computers and appliances. Heaters, lights, pumps and power tools are rarely bothered.

Some things are fussy. Champion lists items that may need under 3% THD:

• UPS-style battery backups, like the ones many people use for computers and modems.
• Pellet stoves. They have control boards.
• High-efficiency furnace control boards. See our gas furnace guide.

Other electronics-heavy loads, like a mini split's inverter board, are kinder to clean power too. See will a generator run a mini split.

The best source is the appliance maker. Champion's own advice is to check with them whether your item needs under 3% THD.

Does THD change with load?

Yes. Champion notes a standard generator's THD varies "depending on load." A spec-sheet THD is one figure under the maker's test conditions. It's a fair way to compare models, but not a promise for every load.

How to choose

• Running a furnace, pellet stove, UPS or mini split? Pick a generator with a stated THD of 3% or less. That usually means an inverter.
• Running a well pump, sump pump, fridge, lights and tools? A conventional generator's power is fine, per Champion. Choose on watts and 240 V.
• Power stations make AC from a battery through an inverter, much like an inverter generator does. Check the maker's spec for "pure sine wave."

Where to find the THD number

Look in the specifications table of the owner's manual, or the spec tab on the maker's product page. It's usually listed as "THD" or "total harmonic distortion" with a percentage, like "≤ 3%."

Words like "clean power," "safe for sensitive electronics" or "low THD" are a good sign, but they aren't a number. If your furnace or UPS maker gives a limit, you need a figure to compare against. If you can't find one, ask the generator maker before you buy.

THD isn't the only "clean power" question

Voltage that sags under a big start-up surge can also upset electronics. Size the generator with headroom. See running vs starting watts.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.