I only have experience with whole house generators using natural gas. Found a few online sources for how to size with lp:
Generac Power Systems - Generator Sizer
and
Build With Propane - How to Select, Size, and Install a Standby Generator
Generac is the name around here but there are others. I would recommend professional installation just because of what is required to hook one up. Our system hooked up ran around $5k.
Our area had no power for around three or four weeks after the derecho in 2012. My family was ok because the generator ran 24/7 until the power came back on. The smell in the neighborhoods from rotting food in bins attracted some bears from the woods. Total panda-modium I tell you.
