nsrdb.nrel.gov
The Foremost Resource for U.S. Solar Data | National Solar Radiation Database (NSRDB)National Solar Radiation Database (NSRDB). What Is the NSRDB? The Foremost Resource for U.S. Solar Data. The National Solar Radiation Database (NSRDB) is a serially complete collection of meteorological and solar irradiance data sets. The data are publicly available at no cost to the user and provide foundational information to help solar system designers, building architects and engineers, renewable energy analysts, and many others to improve and expand solar energy technologies. Access the data sets.
http://nsrdb.nrel.gov/