Speaking of Solar panels, you really need to look for one that is designed specifically for backpacking, as that will be one that is very light and very compact when folded away...The only one on the market that meets both of these criteria is the PowerFilm LightSaver. It has a built in 3200mAh Li-ion battery, that the panel can charge fully from flat in about 6 hours of bright sunlight (pretty comparable to larger, much heavier panels), so it has enough capacity to charge an average smartphone fully between 1 to 1.5 times. It's very compact when rolled up and when rolled out it can be easily mounted onto the back of your backpack so it will charge as you hike...The best bit is that it only weighs 135g (4.76oz), making it one of the lightest solar panels avaiable. They do a larger more powerful model called the Lightsaver Max, but it's about twice the size, and weighs significantly more so it's not exactly what I would call ultralight, but it does have a much larger internal battery. The Lightsaver currently costs around $100 in the US, and in the UK it's about £100.