Fortunately, Wichit Sirichote has come to the rescue with his very detailed Solar Server project that measures the solar radiation striking Earth. It comprises a pyranometer, analogue-digital converter and RS-232 port of a Linux server to capture the solar rays and process them.
These pyranometers may be expensive; get more information on these radiometry devices at the Kipp & Zonen web site.
News source: Gradget Freak: Solar radiation and a delta-sigma ADC.