SOLAR ENERGY BASICS from National Renewable Energy Laboratory

PVWatts Version 1 - PV Performance Model by US Locations @ NREL.gov

PVWatts Version 2

Solar Resource Information Data Center @ NREL.gov

Database of State Incentives for Renewables & Efficiency - DSIRE
DSIRE is a comprehensive source of information on state, local, utility, and federal incentives and policies that promote renewable energy and energy efficiency. Established in 1995 and funded by the U.S. Department of Energy, DSIRE is an ongoing project of the N.C. Solar Center and the Interstate Renewable Energy Council