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Direct beam radiation is solar radiation that reaches the surface of the Earth in a direct line from the Sun. Atmospheric conditions reduce the level of direct radiation, on clear dry days by 10%, on heavy cloud days by 100%.
Depending on how you want to use the radiation depends on how it is measured. For electrical PV systems kWh/m2 and W/m2 (Watts per square metre) are used, where as Btu/m2 (British thermal units per square metre) can be used for thermal energy systems – hot water and space heating.