Solar to hydroelectric dam long term storage is the best bet for summer vs winter, and similar things of that nature. Trouble is you need to ask Scotland nicely

Solar to hydroelectric dam long term storage is the best bet for summer vs winter, and similar things of that nature. Trouble is you need to ask Scotland nicely to act as your big battery for hydro, and there is a point where you run out of places to go. Your next best option is to increase the interconnection to the European grid.