I just received an issue of the NFPA's magazine "NEC Digest." This article, IMO, glosses over the most important part of using ANY kind of back-up or supplimental power supply: the transfer switch.
Generator sizing is also a very important matter. This is one situation where 'bigger' is not always better! As a rule of thumb, a generator ought to be run at about 80% of it's capacity .... it's not really good for the genny to be massively oversized.
As for undersizing... well, that's usually something you can deal with- by pulling a few plugs, flipping a few breakers.
Another item often overlooked in genny installs is the regular 'exercising' of the equipment. The genny needs to operate, under load, at least once a month; weekly is better. The genny should operate under load; this can either be with an automatic transfer of power at the transfer switch, or by using a "load bank" to create an artificial load for the genny.
If one is not willing to 'do it right,' then I say they ought to stick to portable units and extension cords.
