Yeah PowerWall or generator is really the solution for this. They're surprisingly big electric hogs though - our furnace is 500W and is our biggest power draw

Yeah PowerWall or generator is really the solution for this. They're surprisingly big electric hogs though - our furnace is 500W and is our biggest power draw during the winter. A PowerWall will only power this for about a day of continuous usage (realistically heater use won't be continuous because of thermostats and insulation, but it can be appreciable in cold climates).