I would be curious to know what it costs per hour to run a 15KW generator.

Before making a decision I suggest determining your household electricity usage. I know from my own measurements that our household runs on 1,100 - 2,500kw with spikes up to 10kw. I use a product from http://www.wattvision.com to keep track of our usage.

A less powerful generator will cost less per hour to run and give you more run time if you are connected to a finite fuel supply.