</font><font color="blue" class="small">( I have had the Guardian 15KW air cooled generator for over two years. I have had no problems with it, and it has about 100 hours of use so far. Cost was $2995 + about $800 to connect and put on a concrete pad. Keep in mind when sizing your generator that natural gas has less BTU's than propane; in my case my 15KW unit is rated at 13KW with natural gas. We have free gas use on our farm, so I didn't even consider propane. )
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Actually, natural gas is sold by the therm. There are 100,000 BTU's in a therm and 90,000 BTU's in a gallon of propane. Natural gas is used to produce electrical energy now-a-days (instead of coal) and most likely, the only time you will lose gas service would be the result of a third party damage or in a disaster...a tree may topple over and pull the pipe out of the ground with the tree roots.
EDIT: During natural disasters, I have witnessed propane tanks floating off. If you do go with propane, make sure you anchor the tank sufficiently.
DISCLAIMER: I have been in the natural gas business for 25+ years, so I am partial to our product.