I received an e-mail from a company here in Canada to which I'll summerize.

"Thanks for your email.

Our generator heads provide less than 1.5% THD (Total Harmonic Distortion) and the AVR's (Automatic Voltage Regulator) for the 13kW and 20kW is less than .5% + -

I hope this helps."

This seems to be inline with what I require, no?