Hi Mark:
Yes - THD is a "Red Herring" triggered by the O.P.'s question on the topic.
A good quality generator, whether using a traditional 3600 or 1800 RM alternator or a generator driven inverter, will have a waveform that's good enough for just about any purpose. The "gotcha" is that the generator HAS to be "good quality".
One very inexpensive ChiCo inverter generator I tried produced a stepped square wave and had all sorts of issues - connected equipment did not like it at all. Another used a traditional rotary alternator, but had a very poor governor and could not maintain frequency or voltage - yet this two-cycle wonder was praised as a $100 EmComm generator by many in the ham community. Good Grief!
Recently, my cousin pulled his bargain generator out of the box during a recent storm outage - and it failed within an hour (yes, he put oil in the crankcase - that was NOT the problem - and I'm SURE he'll ask me to fix it when I get back there).
But there are other stories of people getting lucky with these cheap products. To quote Clint Eastwood in "Dirty Harry": "Do ya feel lucky....well Do ya?" Some people are - but in a pinch, it's not too wise to count on luck.
My old Generac just keeps on going and going - but it's noisy, burns more gas per KW-Hr delivered than my Hondas and has visible distortion in the AC waveform. With minimal maintenance, it's gone 20+ years. But - it's the LAST machine I'd pull out and use now and is pretty much a backup to the Honda EU6500is. I think I paid about $700 for it, back in the day (about $1,100 or $1,200 today). It wasn't cheap then and it gave great service for years - and is still ready to go. Decent quality, but not top quality.
My little Honda EU1000is is just a little younger and has given me similar service. Way less power, but VERY, VERY quiet and sips gas.
I'm afraid my cousin in CT has used my (fairly new) Honda EU2200is more than I have - he borrowed it recently when his cheap generator croaked. Hope he drains the gas and changes the oil before he puts it back - need to call and remind him - it ran for three days straight.
So - who cares about THD? FIRST find out if the carburetor tends to fall off or if flames are often shoot out of the exhaust (see my previous post) or tends to have "It's dead, right out of the box, Jim" syndrome, or if it can actually hold a stable output voltage under varying loads. There is NO warranty service during an emergency.
Next, figure out what the fuel consumption will be at the expected load and what your expected fuel reserves are likely to be. The bigger the generator the more fuel it will burn. Traditional non-inverter generators are very inefficient at light loads, and their "auto-idle" feature is for no-load conditions only , unlike a decent inverter generator that can run the gas engine only as fast as needed to supply the power demanded while still holding frequency and output voltage.
Then, consider WHERE and WHEN it will likely be used and whether its noise or weight will be a problem in these applications.
Lastly, understand whether 120 VAC, 240 VAC or both will be needed. Not all AC generator provide 240 VAC (MANY do NOT).
Brian - K6BRN