# Why does THD out decrease when THD in increases?

One thing I love about the P10 is the ability to monitor performance over time. So cool. One thing I find curious is the fact that there is a general trend for the THD out to decrease whenever the THD in increases. I would think it’s the other way around! BTW, this tends to hold true whether or not I have significant load on the system. Can anyone explain this?

Also, my THD in averages about 2.2-2.5% with voltage in anywhere from 117 to 124V. How does that stack up to other power grids in the grand scheme of things?

I checked with its inventor, Bob Stadtherr, our chief engineer. Here’s his response:

"Say the incoming voltage is 124 and the THD is 2.2%. That is a 3.3% voltage error and 2.2% distortion. So the voltage error is the dominant problem at 3.3%.

If during high load conditions, the voltage drops to 121V and the THD increases to 2.5%, The voltage error is only 0.8% and the THD is the dominant problem at 2.5%.

So the second case is easier to correct because the total error is less."

Thank you so much for that reply, Paul. That’s very helpful and interesting. I also want to reiterate what a fantastic product the P10 is and how much it has improved my system. Well done!