The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour.

See diffuse insolation and direct insolation.

Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.

Sunlight falling directly upon a collector.

Opposite of diffuse insolation.