Solar radiation that avoids atmospheric scattering and arrives at the earth's surface in an unbroken line from the sun is termed direct, while the scattered radiation that reaches the earth from all parts of the sky is called diffuse. The combined contributions of direct and diffuse result in the total, or global radiation, the quantity of interest for non-concentrating solar technologies such as rooftop solar panels. Like most of the U.S., the various solar properties have been measured at only a small number of locations in Texas. Solar radiation information is also important for building design and improved management of agriculture. Figures 3 and 4, developed by NREL, show the average direct normal insolation in Texas and in the conterminous United States.
Normal insolation refers to the amount that strikes a surface that always faces the sun.
West Texas exhibits the highest levels of direct normal insolation in Texas as well as some of the highest levels in the entire nation. Compared to East Texas, West Texas experiences 75 percent more direct solar radiation. Direct normal insolation is the quantity of interest for concentrating technologies that track the sun throughout the day and intensify natural sunlight to yield very high temperatures or generate electricity efficiently. Since wholesale energy markets are dictated almost solely on price, solar power plants trying to compete in this arena will need to be located in regions with very good direct radiation. To support prospective developments of this type, improved solar radiation data are needed throughout the Trans-Pecos and along the Rio Grande.