Wanted to resurrect this topic after cruising the REI site looking for solar/battery backup to power my new iPhone on the road. I was appalled by the lack of information available to REI's customers and the weirdly sad reviews many of the solar items were getting. People were expecting tiny and cheap solar systems to keep GPS, phones, tablets and even notebooks fully charged. IMpossible.
Be sure you understand some basic math using milliamp hours. The more I research this stuff, the more I am convninced the manufacturers are deliberately obfiscating these figures.
1. You need to know how much power your device requires to charge its battery. (An iPad needs a 2 amp charger, that's 2,000mA, while an iPhone neds half that.)
2. You need to know how much power your device consumes or a genreal idea of how long it lasts doing hard work like GPS or talking.
3. You need to know how much energy your charging device can produce.
4. You need to know how long it will take to charge your device under typical, not ideal, conditions.
5. You need to decide if the cost is worth the hassle.
Whatever you select, be sure to test it before you get out where you depend it.
The state of the art at the moment seems to be the use of solar (or a hub or an AC plug) to top off an auxiliary power pack of AAs or lithium ion cells and use that battery booster to either replenish or supplement your device's battery.
Using MapMyRide or the GPS function on my iPhone sucks the battery dry in less than five hours if I'm not doing anything else. So I need a pwoerpack to keep the thing alive on a good day's ride of 6-10 hours. Then I need to recharge both the phone and the booster pack.