We have a 100lb propane cylinder outside with the line through the house to a portable propane/NG heater in the livingroom. We put in a new TRANE heat/air unit last winter but the power goes off here a lot so we keep the gas for emergency backup or if it’s bitter cold to help out the electric unit.
When I was in Alaska we had a high efficiency Natural gas heater for the house that would shut down in a power failure. I looked on the tag and saw it took 3 amps to run so I set up a car battery with a small 400w inverter for car use for laptops and a small motorcycle trickle/float battery charger.
End of problem, ran fine when the power went out and topped off the battery in between. Just had to plug it into the inverter when the power went out.