What to do about harmonics at the panel?
Pretty much nothing. In these set-ups, my panel neutral (and feed) is already a lot larger than double the size of the #12 branch circuit conductors. Then, there is a short run to the transfer switch, with the monster battery-bank UPS and isolated transformer close at hand.
Get harmonics through those, and you probably shoud re-consider your appliances!
:-)
And, yes, I absolutely agree that twisting the wires helps!
