Solar power or dynamo

[Jefferson Meriwether]

On my tour down to Gibraltar one of the other guys & I had this solar panel charger each.

http://www.portablepowersupplies.co.uk/15w-solar-usb-charger/

I was very impressed with it. It doesn't need direct sunlight; there were a fair few occasions when it still charged my phone/garmin in slightly overcast situations though granted at a slower rate. There were quite a few days when I had my solar panel attached to my handle bars and, even when not directly facing the sun, it charged my stuff up sufficiently. If you decide to go down the solar panel route I'd definitely highly recommend this one.

 

[willem]

It primarily depends on the GPS that you have. I have an Etrex 30, and it is not only light and relatively cheap, but on two modern top quality rechargeable AA's (I use Ansmann 2500) it will last for 4-5 days. So on shorter rides all I take are a few sets of spare rechargeable batteries, but not even a charger. If you still run out, you can always buy some disposable AA's.
If you turn your phone on for only a few minutes each day to send and or receive a text message a phone will last for a week or so. Take a small battery pack that you can charge anywhere without worrying if it gets stolen. You can charge it with the same usb style charger that you use for the phone.

 

[flaluminum]

In my opinion, power bank is definitely the way to go. Can charge cell phone 10+ times and there's always power at enough stops to recharge it.