Keep in mind that you don't need a traditional electrical battery for heat - storing heat directly is extremely cheap, you just need insulation and thermal mass

Keep in mind that you don't need a traditional electrical battery for heat - storing heat directly is extremely cheap, you just need insulation and thermal mass (i.e. sand/rocks, any old crap will do). There are a fair few systems that use easy summer-solar to generate heat, store it for months at a time and then release the heat in winter. It scales up really well thanks to the cube-square law, which is a euphemistic way of saying it's hard to make a viable system that's really small. But if you're off-grid because you're in the middle of nowhere, then you can spare a few square meters anyway.