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angle of incidence
The angle that a ray of sun makes with a line perpendicular to the surface.

A surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.

This entry is located in the following unit: Photovoltaic Conversion Efficiency Terms + (page 1)