[lightbritefight]

An ideal side effect of over overbuilding solar 5x for winter is that you could then use that spare solar in the other 3 seasons to make green hydrogen, an energy intensive process, to power retrofitted gas plants. You could store and ship excess gas as well, creating an adhoc "green energy interconnect" with other nations.

[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!