+1 on Solar, With the addition of a secondary battery- 2 separate items.

That way the sun is used to charge the battery in advance-- If you HAVE sun, charge your phone directly from panel, If night, charge from battery.

I think I got a 5 watt folding camping panel (just USB outs, about 7 x 10 folded up) for 29, and a 12,000 Mah battery for 14.

Heck, the battery even has an LED flashlight on it....

I can charge a single device in light clouds from a 5 watt panel, or 2 devices in the sun.