This map divides the world into five solar performance regions
based on yearly averages of daily hours of sunlight and ambient
temperature. Each specific site will, of course, be different
Also, local weather conditions and seasonal changes can
significantly affect the amount of sunlight available.
http://www.oksolar.com/abctech/world_solar_radiation.pdf
USA:
The average daily solar radiation available in a few
representative cities of the US. It shows a clear variation in
the availability of the solar resource in different areas of the
country.
(info provided by NREL).
