NREL is the only federal laboratory dedicated to the research, development, commercialization and deployment of renewable energy and energy efficiency technologies. Backed by 33 years of achievement, NREL leads the way in helping meet the growing demand for clean energy.

NREL developed PVWatts - http://www.nrel.gov/rredc/pvwatts/.  Use this tool to calculate how much energy will be produced by photovoltaic solar panels in a geographical area. It takes into account the amount of sun in the area and average electric rates.