/diy/ - Do It Yourself

I'd like to plumb a propane tank into my gas line.

I have natural gas in my city and I was to be able to have a backup supply in case the power goes out for an extended period of time my backup generator would continue to work, I figured I could just add one of these tanks between the gas meter and my house to act as a sort of battery.
My gas bill would be super high to get it filled up initially, but then from there on I would be okay.
I see these for sale/for free occasionally on craigslist.
Is this a viable idea? Thanks