Solar Power Backpack

I'm off to Belgium camping in August.

Now I know for a fact my iPhone battery wont last 5 days without charging.

So ive been thinking of getting a solar backpack or something like that.

Ive seen this on the apple store

http://store.apple.com/uk/product/TX...co=MTY3ODQ5OTY

Any recomendations ? or is that my best choice