I've installed some of these whole house generators for friends, in my former life I was in the electrical business for 14 years. So a few things you need to know, if you take Generac's information regarding a 22kw generator on propane it will burn about 2.1 gallons of propane an hour at half load. But there are things you have to consider about propane tanks, for instance, if you have a 250 gallon tank, first they will only fill it to 80% full, then you can on use it down to 20%. So if you take 250 gallons less 40% that leaves you with 150 gallons usable fuel. Divide that by 2.1 gallons an hour and it will give you 71.42 hours of run time, divide that by 24 hours and it will last 2.97 days. And like someone else said in a hurricane or a massive ice storm like the one that hit the Mississippi River Valley from Memphis to New Orleans on both sides of the river for miles east and west in 1994 were some people were out of power for a month or so, getting someone to deliver diesel or propane probably isn't going to happen anytime soon. Natural gas is best if at all possible.