N900 draws like nuts. Under heavy use, it drains faster than USB can charge it. And USB drives 2.5 W (5v 500mA). Under less than desert conditions it might be that even low usage drains. N900 has 5 hour talk time under 3G. Add speakers or BT and solar can't even maintain a call. Plus, if you talk you probably have poor incidence and a hand over it. No way around it for now. You need big panel and not glued to phone.which makes it less portable than a second battery and an emergency pack.