Ahhh.... So it's say a 320C/400A service? (2)200's in a meter/main combo w/ (2)200 lug panels, or a meter pan and (2)200 main panels?

Sounds as if one whole panel is general lights andrecept load? They may be lightly loaded, but the gennie should be sized for the load or in the very least a high demand of that load. Otherwise a small gennie would be defeating the purpose of being there when you need it if they are out there changing fuses, or flipping breakers. A generator salesman is going to give you the opinion that it needs to be sized a 100-150% of the load - but that is not nessesarily true...
http://www.generatorjoe.net/page.asp?id=40 http://www.dynagen.com/page.aspx?id=96259

Motor starting can drive it up a bit, but most are intermitant... IMO But you can guarantee that if everything else is off, what can be on - will be on.... So 100%+ is good. But my own opinion would be ~80% demand plus motor starting. Diesel likes a load.... Gas doesn't.

One of the nice things about an ATS, or more correctly certain ones. They can excercize the generator and load bank it on the house loads.

Another part of this is who gets to maintain it? It will need oil changes, and all things automotive, spare tire rotation. [Linked Image]

Mark Heller
"Well - I oughta....." -Jackie Gleason