Anyone know anything about residential generators?

We bought a 15 KW Generac generator for our house in the event of loss of power. We only desire to run small portions of the house at any one time, but the electrician (who I feel is an idiot) thinks it cannot be doen without getting a larger unit. (Think tons more money)

I have talked to some who say it should be no problem, but I am unsure. Any help? Anyone?

Maybe I should take up electrical engineering as a hobby!

Anthony