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According to measurements from Lawrence Berkeley National Laboratory, the average cell phone draws 3.68 watts of power from the outlet while it's charging and 2.24 watts when charged.
A typical Mini Solar Panel operates at 3V and provides 20mA (see how to make a cell phone charger). One One can buy off the shelf solar cell phone chargers, for example the SolPro that in direct sunlight can re-charge its battery (a Lithium-Polymer battery with safety circuitry and capacity of 4500mAh enough to keep a smartphone going all day) in 4.5 hours.
Relevant papers
An analysis of power consumption in a smartphone, A. Carrol and G. Heiser, 2009