@epierre

I recommend a single or double USB 18650 battery bank. You can get a solar panel and attach the leads to a 5v USB regulator. You can use a USB cable to connect the solar panel w/ 5v regulator to the input on the 18650 battery bank.

The output of the battery bank can be connected to the micro controller.

http://www.ebay.com/itm/Mini-Portable-Power-Bank-Mobile-USB-18650-Battery-Charger-Keychain-f-Phone-MP3-/161300528675?pt=US_Cell_Phone_PDA_Chargers&var=&hash=item258e42be23

http://www.ebay.com/itm/Battery-Pack-for-2x-18650-Rechargeable-Battery-Mobile-Power-Bank-USB-Output-5V-/400729621411?pt=US_Cell_Phone_PDA_Chargers&var=&hash=item5d4d58c3a3

http://www.ebay.com/itm/USB-DC-7V-24V-to-5V-3A-Step-Down-Buck-KIS3R33S-Module-Arduino-than-LM2596-/201080977521?pt=LH_DefaultDomain_0&hash=item2ed15c3871

http://www.ebay.com/itm/5W-epoxy-resin-solar-panel-5-watt-12Volt-solar-battery-charger-diode-easy-carry-/171472266424?pt=LH_DefaultDomain_0&hash=item27ec8b24b8