You can also have a manual transfer switch installed and a generator outlet installed on the outside of your house. It's much safer and less expensive than an auto transfer switch and NG generator.
You would still need a portable generator to use with the manual transfer switch. But your looking at $1000 - $1,500 plus installation instead of $6-7k.
http://www.grainger.com/Grainger/RE...2KEP3&ef_id=UIrSPAAAXcvtm1d5:20121105141450:s
Be careful with the generac 20k. on amazon.com there is a lot of bad feedback. If I needed a large one I would definitely go with a kohler. It's more money but the motor has a better design.
thanks for the heads up.....I already got their catalogue and will be ordering a Generac NG house unit soon...