This is a very good idea to save energy. Although this concept is not in market this time but it’ll surely make you tension free. Now you need to worry if you had forgotten batteries at home or they are discharged. Only the problem is “weather”. But in this case you may use your regular batteries. You may use solar strap as alternate. If you search over the net more about solar designs you’ll get more ideas like this. Many of them had implemented as well. Like folded & micro solar panels which may be suitable to charge your mobile phones or i-pod.