Brother has had a Generac auto stand-by for 5 yrs now. Works great. Mine will be installed in a couple weeks. His runs on LP, mine will run on NG.

Regardless of how you add up the watt/amp load, I think you'll find the largest air-cooled is around 20KW. Getting into that "mid-20" range requires large water-cooled engines...meaning more $$. I suspect a well maintained stand-by generator should last 30+ years.  You'll probably not have a lot of running hours at the end of 30 yrs, though they do excercise themselves weekly.