Any fat, or shortening will work as a makeshift candle. Though personally as other posters mentioned, you're burning something valuable in a long term crisis. As preppers it's good to be able to have different options. But if you are huddled around a jar of shortening slowly burning, you're preps kinda suck. Kerosene, and paraffin oil is fairly cheap, and you can run a long time off a gallon. Coleman lamps are still the kings of light output, for heavy weight camping, though I'd want a propane one if it was for inside. For just pure light, some of the better LED headlamps, work lights and flashlights put out some great numbers and are pretty frugal on battery power.
For warmth, if a wood stove or getting the furnace running isn't an option, bigger inverter could run a good space heater, or maybe two. But even a couple won't do much in a big space. But gas to run it, is cheap, and it'll help. Especially if you shut off rooms and have heavy curtains. The colder it gets, the more challenging it will be. Well below -40 is a different deal than barely freezing.