One thing I am definitely not familiar with newer UPS units. I guess some of them are pure sine wave now. It's when a high RELATIVE TO TOTAL CAPACITY inductive load is fed by a generator that it's true colors of a distorted or not waveform become obvious. Unless I am mistaken any thyristor based inverter reverts to square wave when approaching full output. But this comes from years ago Even the interstate DC intertie was/is hugely protected against overload. Man I am getting utterly obsolete on some of this stuff...