My first choice is the USB and dyno hub setup (probably B&M Ewerks), they work rain or shine as long as you are moving. As a very close second would be the rechargeable battery packs, third plug in at lunch time or during a break, solar would be my last choice. I have used a variety of solar powered devices over the years, it works great for water heating and static locations for power generation, not so good if you are on the move.

Aaron