How much power does it take to keep an oil furnace running? Probably not a very large portion of your normal total usage. Set it up with the option to run off a backup power source and get one large enough for that projected need. Then you’re at risk only if the supply chain for replacement oil or generator fuel goes down too long.
Typical oil furnace draws up to 12 amps (2 amps for burner pump/blower and 10 amps for duct blower); that’s about 1,500 watts. (Same as a typical space heater.)
Depends on the type of oil furnace. Is it forced hot air which requires a blower for all the heating vents or is it hot water based (boiler) pumping water. Either way you need power.
Having gas makes more sense. You have a gas stove it serves two purposes, heat and cooking.