Irradiation

Irradiance is an instantaneous measurement of solar power over some area.

There are four different types of irradiance:

• Direct Normal, which is measured by sensors that “look” just at the solar disk;

• Global Horizontal, which is measured by sensors that “look” at the full sky dome, because their sensor is horizontal

• Diffuse Horizontal, which is measured by instruments that look at the full sky dome EXCEPT for the solar disk (that's done by obscuring the solar disk with a black ball or disk, so that the direct normal light doesn't reach the sensor - however usually the ball obscures part of the dome that is just around the solar disk, i.e. the circumsolar area)

• Global Tilted (or Irradiance in the Plane Of the Array), which measured by the same sensors measuring Global Horizontal, but they are tilted at some angle, which usually matches the angle of the solar photovoltaic array we are interested in).

The unit is W/m2.