In reply to Patzl:
I've got a Goal Zero Nomad 7 and Guide 10 pack. Works great, if you look up and have to squint your eyes even slightly, then it's light enough to start charging.
Does take a while to charge the pack fully (about 8 hours). But that will charge my Samsung Galaxy S3 twice fully. So even a party charged pack will recharge my phone.
I got it off sports pursuit for £50, not sure i'd pay the RRP of £100. Maybe worth around the £70-80 mark if you can find it for about that?