*From what I have as option (but there must be more since we have a hardware category !):

Li-On 3.7 V batteries
LiPo 3.7 V batteries
solar pannels
I have seen some kitts such as the Seedstudio Stalker which includes board, battery, waterproof box and an arduino and xbee radio for €50.

Another kit has a solar pannel and a 2300mAh battery, but also many cheap solar pannels, some that can fit in the project box on this site.

I will try some BRC 18650 Battery 5000mAh 3.7v, but I would need advice on how using a solar pannel with batteries...

Thanks in advance for your ideas,*
So what did you choose epierre