As a rule of thumb NG generator produce ~80% of the power using the same engine as it does on liquid gas.
Also depending on the reason for electric outage you may not have NG feed to the house.
The installs we have use NG as primacy, LP as secondary (while expensive does not go bad) and liquid gas. Regardless of the type of fuel sources can be interrupted.
The key is the duty cycle and requited operating time.
Per NotMe:
Thanks for the succinct and definitive reply.
What is your experience with how long propane tanks themselves last? i.e. they are steel and steel rusts.... Anything over 10 years would effectively be "forever" in our case.
What I'm thinking is something set up for nat gas with a manual cutover switch to a tank of propane in the event that worse comes to worst.
OTOH, that would seem to require a longer run for the nat gas line. Without the propane backup, the gennie could be as close to the house as zoning allows. With propane backup, I would think it needs to be away from the house (as in the garden shed or something) for safety reasons in case of a house fire.
Or am I over thinking this?
You apparently have nat gas. In 30 years with nat gas, I've had many power outages from a few seconds, all the way up to Hurricane Sandy, where power was out in much of the region for up to a week+. Never lost nat gas once.
I guess whether you're over thinking it depends on what exactly you need the backup power for, what kind of worse case event you want to protect against, and the liklihood that nat gas would go out at the same time. I guess in an earthquake area, there the gas system would be more vulnerable. But in most of the prime earthquake areas, eg CA, the climate is such that needing the generator for heat isn't nearly as great as it would be in WI, so one important need is lessened. I would think for 99% of the folks using backup generators, having nat gas as a fuel would be more than sufficient and the cost, having an ugly propane tank, etc wouldn't be worth it.
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