As I am currently looking into buying an electrical car, I was considering doing (almost) exactly this : Installing solar cells on the roof of the house to charge the car. It wouldn't even take that much solar cells; 20 square meters would charge the car in a reasonable amount of time. Free energy, right?
While considering the idea, a fundamental problem stuck me: Most of the time when the Sun shines, the car isn't parked at home. It is either parked in front of my office or, when I'm not working, I'm driving somewhere else, enjoying the Sun that could have charged my car. The solution to this issue was to add batteries to the concept, in order to store the Suns energy as I am away and transfer this energy back to the car at night when I am home.
Considering the car has a capacity exceeding 20 kWh, the battery solutions becomes extremely expensive - as expensive as the car itself actually (if not more). Without the battery, it's a nice expensive systems that will produce a lot of power when I don't need it. It's always possible to sell back the excess power to the utilities, but you get a loss let out of it this way and it makes your life quite complicated.
Forgetting this fundamental limitation, after doing a lot of calculation, it turned out that it would take over 20 years to amortize; and I doubt the battery system would last 20 years under the kind of stress it would be put too (nearly daily full deep cycles). And this is assuming the normal electricity prices. In fact, the charge stations are highly subsidized and your are basically paying the price large industry would pay for electricity. Suddenly your amortization period goes up over 40 years.
It's not (yet) worth it, although the technology is actually there and ready.
Conclusion : Power accumulation solution in the 20 - 40 kWh range are too expensive and power is too cheap.