Nice build on using an arduino to controll the charge to batteries for a solar array. Gives a nice display on stats too. Why would you do this? Well its a cheaper way to get information and control the charge (I think $50 in parts?) than buying the commercial version. And you could add onto this one (network the stats for example).
http://www.instructables.com/id/ARDUINO-SOLAR-CHARGE-CONTROLLER-Version-20/