I use an external outlet rechargeable battery. For several tours I've used a 7,500mAh unit to run an Vista HcX all day for more than a week plus charging my phone some. Last tour I ditched the GPS and went straight iPhone-GPS I got about three days per fully charged unit. This year I've upgraded to a 15,600mAh unit ($24/10 ounces).

My experience a few years ago with solar says it's not ready for prime-time touring. BUT i am looking forward to when it is.

Dynamo hubs seem pricey, lunky, and kludgy for the usb charging I'm look for.