Hackaday has a cool post on Ladyada’s video on solar charge control analysis.
http://hackaday.com/2010/08/07/solar-panel-charger-analysis-with-ladyada/
The unit itself is built around an Arduino and can log the statistics to an SD card, show battery voltage, panel voltage, and current from panel to charger. You can see in the video above how she uses this to refine her design in real time for optimal results.