Portable indoor heater for emergency
You could install a direct vent, wall mounted, sealed combustion, heater that requires no electricity and can be fired with your existing natural gas service or a separate propane source fuel. They are readily available through a myriad of supply houses like Home Depot, Lowes, Graingers, E'bay, Amazon and local contractors. They come in almost any size you would want. For more information just use your computer search engine. This of course would not be portable but would give you heat in an emergency and would not need any electricity unless you added an optional blower fan.