I've got this solar panel and it works well when it's in positioned so that the sunlight hits it directly. When I fix it in place on top of my rear rack, it doesn't produce nearly as much, unless the sun is very high overhead. I'm not an electronics guy so I can't express it in numeric terms. Anyway I'm still glad I have it. I'm in Australia and it's summer at the moment, so I take long afternoon breaks, and then I can place the thing on the ground and angle it to the sun and it works pretty well. Definitely better than nothing. Just don't get your hopes up too high in terms of amount of current you'll be able to generate from solar.

I've also got a Schmidt SON hub (with an e-Werk and cache battery). I use a Garmin Oregon 450 GPS which uses 2 AA batteries, and I carry four rechargeable AA batteries. Each day I swap the pair in the GPS with the pair in a USB AA battery charger plugged into the e-Werk cache battery. I usually don't have much charge left in the cache battery to charge anything else. I get more power on days when I'm riding faster and longer distances obviously. The light on the cache battery (to indicate incoming current) only lights up when I'm going faster than about 7.5 km/h, and there are some days, like on the Munda Biddi Trail, when I'm not spending much time going faster than that.