Not sure how it performs yet, still to try it out in the wild, but have a look at Solio hybrid charger. perfect for the british summer as it contains it's own rechargebale batteries which can be pre-charged from the main, and topped up in the field, or can be charged from flat in the sun. By putting a pencil through the hinge in the middle it can also be stood up and directed at the sun for efficiency. it's quiet pricey, but from a full charge it will apparently fully charge a mobile phone 4-5 times from flat.