I've had a Generac for about 5 years, 9KW, runs off my Natural Gas. Good friend of mine is a electrician, advised me to stay away from the 24KW whole house unit, said you really don't need one. When was the last time every electrical devise in the house was running at the same time.
My 9KW runs my well, furnace, refrigerator, freezer, fireplace, kitchen appliances, family room lights and TV, bedroom lights and Tv , my office and computers. I've never had all of them running at the same time, so 9KW is really sort of over kill.
My main concern was power lose in the winter, no heat, plumbing freezes and a dark cold house. Didn't bother hooking it up to the AC, we plug in the fans the few days we've been without. A generator is nothing more than an Insurance Policy, it sits idle about 90% of the time, but the few times you need it, it sure is nice, envy of the subdivision when the light do go out.