In Oregon they used to require a secondary heat source in every house stick-built. Very sensible, as power can go out & sometimes for days at a time. My house here in Idaho is electric only. I do worry what I’d do if the power were out for any length of time.
Where we are- we have only one source- fuel oil burner- when electric goes out, we fire up the generator and run a room heater when it’s not too cold, or the house furnace if it’s real cold- we just have to be sure to have several cans of gas for the generator as outages in the sticks can last days sometimes-