The solar panel charges an internal battery slowly, then you transfer power from the internal battery to your N900 battery. I'm pretty sure that is how they all work. Otherwise it simply wouldn't work for electrical reasons, or you would need a BIG solar panel.

I would just buy a 2nd battery or a car charger. The only reason I would consider solar is if I was going somewhere that didn't have electricity. Otherwise its just not efficient. You'd be better off with something like this:

http://reviews.cnet.com/4520-11288_7-6427792-2.html