[gridspy]

That's probably assuming a solar system sized to cover typical summer energy usage. You can simply over-provision solar until you have wasted capacity in summer and little to no storage requirement in winter. Then it's just a tradeoff between battery and solar costs to find the best price point.

Also this calculation probably assumes no baseload power imported from the grid, where means such as wind and tidal power work year-round and help offset the need for batteries.