A couple of weeks ago I started working with a 12V solar setup. I tried to budget this at $200 or less as a start, but it seems like it is a nickel and dime process that is getting ready to pass my budget limits. The system consists of a 12V 50W solar panel, PWM solar controller, 35Ah SLA battery, 10 Amp 12V to 5V regulator, Activity Board/RPi combination and the nickel and dime stuff like wire, connectors, nuts and bolts, ..., etc.
I chose the 12V system because the essential parts for this are readily available. If you go with something other than a 12V system you will probably have quite an interesting time trying to get the system to work like it is expected. For me their is no "green" philosophy or "saving the earth philosophy" involved, just trying to see if the sun can do some work for me, other than giving me a sunburn or stroke.
My early observations, I have to start thinking about implementing devices to replace the power hungry Activity Board and RPi. Yes, I was surprised to see how much power they really use when you can actually track it with the 12V battery as the power source. The other item that is pretty hungry is the PWM solar controller, I was surprised about that also. I am only talking about a volt or two, but when you hit a non-charging day, you will run dry very quickly.
I am now in the process of adding another battery to be in parallel with the other battery to see if the solar panel can deal with the added work, I have a feeling that I might have two upgrade the panel to a 100W version, but not sure. I am also trying to setup the Activity Board ADC component to have it give me data about what the two batteries and solar panel are doing. The worse part about this is getting the wiring right, I already let out some magic smoke that was within the wire, actually now it is only a lot of black wrinkled outer coating that is left with the cooper innards of course. More to come.