About Photovoltaics


Turning Sunlight Into Electricity


Light and the Sun: The Solar Resource


When sunlight reaches the Earth, it is distributed unevenly in different regions. Not surprisingly, the areas near the equator receive more solar radiation than anywhere else on Earth. Sunlight varies with the seasons, as the rotational axis of the Earth shifts to lengthen and shorten days with the changing seasons. For example, the amount of solar energy falling per square meter on Yuma, Arizona, in June is typically about nine times greater than that falling on Caribou, Maine, in December. The quantity of sunlight reaching any region is also affected by the time of day, the climate (especially the cloud cover, which scatters the sun's rays), and the air pollution in that region. Likewise, these climatic factors all affect the amount of solar energy that is available to PV systems.



Although the quantity of solar radiation striking the Earth varies by region, season, time of day, climate, and air pollution, the yearly amount of energy striking almost any part of the Earth is vast. Shown is the average radiation received on a horizontal surface across the continental United States in the month of June. Units are in kWh/m2.

For more-detailed solar resource information, contact the National Renewable Energy Laboratory's Renewable Resource Data Center


U.S. Department of Energy Photovoltaics Program



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