You didn't take into account that a well built house reduces heating and cooling demands by more than 70%. We're talking hundreds of kwh a year, if not thousands, for each houses

You don't need a power grid to run what basically amounts to a bathroom fan. And guess what? Even if you're completely out of electricity you can still open the windows.

The bottom line is that at best you save 70%+ of heating/cooling energy and at worst you're just back to square one temporarily