Solar Photovoltaic Energy devices use semi-conducting materials to convert sunlight directly into electricity. Solar radiation, which is nearly constant outside the Earth's atmosphere, varies with changing atmospheric conditions (clouds and dust) and the changing position of the Earth relative to the sun.  Commercial buildings tend to be well suited for PV because they are typically large and flat with minimum shading or nearby obstructions.  Whether commercial or residential, almost all U.S. regions have useful solar resources that can be accessed and used to save energy.