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Total harmonic distortion (THD) of portable generators

BuildingNewb | Posted in Mechanicals on

I’m looking for some insight on the total harmonic distortion (thd) metric for portable generators. I currently own a new Westinghouse WH7500E Generator which is 7500 continuous watts and 9k peak watts. However, it has come to my attention that it has a 9% thd. As such, I’ve become a bit paranoid that it will do damage to the electronics in my home and fail to adequately operate the variable speed blower on my furnace.

My panel is wired with a 30amp inlet and an interlock kit.

My question is how a manufacturer can state a specified thd figure when it is pretty much all based on the rpms which are not electronically governed like standby or inverter generators.

I was considering buying a Briggs and Stratton 8k running/10k peak model because they state a thd of 3-6%.

Isnt thd largely dependent on different loads initiating and the rpm engine at that time? I don’t understand how Briggs can claim a lower thd number than the Westinghouse model. Am I overthinking thing entire thing?

Lastly, whether I keep the Westinghouse generator or move on to a Briggs, will the installation of an aftermarket natural gas kit have a material impact on the frequency or thd of these generators? I know there’s an adjustment for the air/fuel ratio but I don’t want to install it if it will negate the manufacturer’s stated specs for thd.

Thanks in advance for the information!

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  1. GBA Editor
    Martin Holladay | | #1

    Building Newb,
    All I know is that the inverter-based generators produce a cleaner power output than the non-inverter units.

    I found this on a relevant web page: "Since most [generator] manufactures don’t publish this data [on total harmonic distortion] unless it becomes a selling point, you can expect the 9 to 10% range to be standard. Just note the lower the cost the more likely the THD is to increase. Standard units are going to be fine for your basic backup needs for appliances to keep you in light, the food cold, and basic small appliances. Clocks and other components may not function as well unless your using higher quality power from inverter systems or very well built standby generators."

  2. user-2310254 | | #2

    This generator web site addresses your THD question ( If you don't want to replace the generator, talk to the dealer/manufacturer about whether you could put the sensitive electronics behind a high-quality UPS or power inverter. These devices are generally designed to handle power fluctuations, and the on-board batteries (at least on better units) can supply attached electronics with a pure sine wave.

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