My thoughts are coming from this
PDF from Generac on the 17-20kw models (page 3 gives the alternator spects), They rate there generator @20,000 watts and it has a 1 PF so there is no loss from that as a matter of fact a quick look up of several generators I could not find one with a lower then 1 PF???
I don't see anything in the NEC that requires us to size a generator any less then 100% of its rated output, the only requirement is for conductors which are required to be sized @125% of the rated output and I think that is only for 700 and 701 installs not 702, but I could be wrong on this.
a 20kw generator has an output of 83.3 continuous amps, this output is reduced to 75 amp continuous when used on natural gas not because of the alternator but because of the ability of the engine to produce the required horse power, most generator manufactures use the same alternator for both NG and LP, the selection of LP or NG is field changeable for most manufactures so even if you are running it on NG you still have a 20kw rated alternator, but we must follow the requirements to only ATS into what the generator is rated for or there will be problems as the #one problem would be the warranty if it was found that the generator was being over loaded , yes the Nexus has a black box that would tell on you when they send a person out to administer the warranty just like the new cars.
Almost all generators do have a surge amp rating that allows for the start up of motors such as air conditioning systems and such but this is only for short periods of time and while a 20kw can handle in some cases up to 3 such A/C systems it would not start all 3 up at the same time, I have installed time delays on A/C condensers to over come this problem and it worked great and saved purchasing the Nexus smart control transfer system that basically does the same load shedding there are other options to load shedding that can be done with current relays to prevent two heavy loads from running at the same time.
But basically if the generator is rated for 20kw or 18 kw in the case of NG then that is what we must size the load for, then up size the conductors for the 125% requirement if it does apply to 702 installs?
Now I would find it a good thing to leave a little head room and maybe factoring in a 20% head room might be a good idea but I can't see this as enforceable?