Solar Thermal Water Heating Systems use the sun's energy to heat water for use by homes, commercial buildings and swimming pools. While some solar collectors look like solar photovoltaic panels, they are quite different. Solar thermal panels do not generate electricity, but simply absorb sun's heat.
NREL: Solar Radiation Maps: http://rredc.nrel.gov/solar/pubs/redbook/
(These maps show the general trends in the amount of solar radiation received in the United States and its territories.)