You can use a solar panel along with a charge controller for maintaining a battery bank. Some users do not realize that incidence is important and does change over seasons. That is unless you happen to have the panel located on the equator then once you setup relative to solar noon for maximum output for the panel then no changes necessary. Sadly not everyone lives on the equator, to setup your panels angle for your latitude and longitude so as to have maximum incidence for that period. Most simple setups work best by using solar noon as the set point for the location.
Of course if you happen to have a active panel that tracks the sun then you would setup initially for max incidence then you will provide the angular changes for the seasonal periods for your latitude & longitude. Some active controllers do provide program controlled angular adjustment for the seasonal periods while others may just have single axis control. This of course will cost you more to have multi-axis control. Static systems are a lot cheaper.
To me a static system would serve the RaspPi well once you size the Solar Panel system for the required loads.
I have used Solar panels for remote logging systems without to much effort. Be sure to know the power necessary to run your equipment. Do not forget to include the power necessary for the Solar Panel controller or Charge controller for a active system.