The question is, if complete energy autonomy is even that desirable. From an economic perspective I think it makes sense to use the grid as a kind of battery. Sell surplus energy to the grid in summer, buy it back in winter (ideally renewable) and let the utilities deal with the headache of balancing supply/demand and storage.

Also, solar panel efficiency is still increasing at a steady pace. A gain of 50% compared to the average efficiency of currently sold panels seems to be achievable in a couple of years. 20% vs. 30% could be all you need to improve your calculation.