dear Bill,
Sorry I took this long to reply......just noticed it.
If I remember correctly, it cost me about $3,000 for the unit......they may have gone up some since.
you can buy a guardian unit online at some good prices. it depends on how big a unit you want.....10khw or 13 kwh , 15kwh, etc,etc. I'd have to go to their website to see the latest models.
I can tell you that it was easy to install myself. The instructions were very good, and if you are comfortable working with electrical, gas lines, etc, it was simple.
As to how long a 500 gallon tank would last, I'm not sure, but if I remember correctly, it was well over a week......depends on what other things are using the propane at the same time. If my boiler was also heating the house, it would not last as long. I do not believe the generator engine uses that much propane. Your propane company should be able to tell you that info. If not, Guardian or a authorized installer may know.
We lost power for about 5 hours Tuesday night.....it is kinda cool to be the only house in the neighborhood with lights on.
Frank D.