Trust voltage output setting or meter?

My P5 outputs ~119.3V when set to output 120V. Should I set it to output 121, in which case it outputs ~120.3?

Not sure why they are off. Is the meter not calibrated correctly? Shouldn’t the software set voltage to match the meter and your desired setting?

My incoming voltage is usually only 1-4V below 120, so changing the mode to voltage regulation vs distortion reduction doesn’t make a difference.

The meters should be fairly accurate. It could be that the P5 has a hard time regulating to that specific voltage while handling all the other aspects. If setting the unit to 121 gets you closer to 120 that’s what I would do personally.

120V rms (root mean square) doesn’t rms value depend on a sin wave. If you’re not outputting a sin wave then maybe the value is miss read rms = 0.707 of peak for a sin wave iirc

RMS does not depend on a sine wave. The RMS of any wave form can be calculated/measured.

Edit: It just occurred to me you may be referring to meters.

An analog meter calibrated for sine-wave voltage or current will not indicate true RMS when reading a non-sinusoidal wave. This is because any wave shape other than a pure sine wave has a different ratio of RMS and average values. (For a sine wave, the value is 1.1107 which is 0.707/0.636. The RMS of a sine wave is 0.707 of peak value. Its average is 0.636 of peak.)

More sophisticated digital meters can calculate and display a true RMS value, regardless of the waveform.