I've got a couple of solar chargers/batteries that I use for everything handheld (Phone, Books, DS) I Just leave it by the window and plug in to the device when I need it.
The batteries are both about the size of a smart phone (If a bit thicker) and while it takes a good 12 hours+ of sunlight to charge the internal batteries (3500mAh) it only takes a couple of hours to charge the handheld from the charger (Plus the chargers are small enough to take with me if I notice at the last minute that I'm low on power).
And all this in sunny england
Mind you I don't use the phone or DS that much so only need to charge them once every couple of weeks.