It seems like some of the responses here are missing the point. Sure, we get to play Angry Birds the whole way on a flight to Australia. But countless people in areas where electricity is still hard to come by suddenly gain the ability to use cell phones and computers, while only needing to charge them infrequently.

Given the impact of mobile devices we've seen in Africa so far, this sort of innovation would have a huge impact.

That said, I don't agree that Bill Gates is necessarily the best person to go solve this problem, nor that this would be an obviously more optimal area for him to invest in than those he's chosen so far.