[Qwertious]

IMO the best use for excess summer solar for winter use is directly storing heat in a heat energy storage system (HESS, a fancy word for a bunch of sand/rock wrapped in a meter or two of insulation). Dump in the heat through resistive heating, use the heat directly for heating in winter. Batteries are expensive, but thermal mass is literally dirt cheap and scales up expoentially well thanks to the cube-square law. Just store and use the heat directly!