In this next year, I've been thinking of adding a natural-gas backup generators from Kohler or Generac, but in 2011 New Mexico had a pretty widespread natural gas failure due to cold weather conditions, and when natural gas fails, it takes a while to recover. But, I'm thinking all we really need is for *either* the electric or the natural gas to work, to keep a house warm or cool. That is, if the natural gas fails, you plug in electric heaters (we have two of those.) If the power fails, you run the generator. If both fail during a freeze, you drain your water pipes and check into the Phoenix Four Seasons. Or fly to Cancun. 8-)
When we built the home in 2012 (HERS index 48) I deiced not to have any natural gas, all 100% electricity. I did sub in a connection if we ever sell the home, some really want a gas stove (note, Induction is SO much better). I'm not missing gas whatsoever and yes, it it can fail, worse blow up.
With a 250 gallon Propane tank, our generator could run for weeks, but again, it's not a whole house system, just the critical stuff. Our needs to heat and cool is modest because we have geothermal (ground source) radiant heat and cooling and a very efficient heat pump.
BTW, the Generac electrician seemed to prefer Kohler.