This is kinda neat, somebody's tacked on a solar panel to a Bit Racer:
http://home.ust.hk/~bcandyp/hof20.html
Don't know where this guy is getting his solar panels from, but I imagine if it works on a Bit Racer it could easily be adapted to a Bit Char-G.
There is one setback, however, it only runs 12 feet on a single charge. So it's not practical yet and you're not going to rush to build it, but it's interesting to look at anyway.