The Global Horizontal Irradiande (GHI) measures the total amount of light received by a square meter on the ground.   Solar irradiance on a PV panel in the Plan of Array (POA) is a combination of direct solar rays, diffuse irradiance and albedo (irradiation reflected by the ground).
The direct irradiance on a panel or Plan of Array (POA)  is the irradiation coming directly from the sun multiplied by the cosine of the angle of incidence (the angle between the direction of the sun and a vector normal to the panel).

Irradiation
Direct and Diffuse Irradiation on the ground

In addition to direct irradiance, the PV module receives diffuse and reflected irradiance, which are function of the module tilt angle, the soil reflectance factor, and other parameters.

IrradiationModel
Direct and diffuse irradiation on a tilted module

The following figure illustrates the average annual GHI and POA irradiance for five locations in BC, Canada.  Irradiance on trackers (Orange) is much higher than POA on fixed systems. (Yellow).  GHI is lower than both .  It shows the importance of setting the panel at an optimum angle or using trackers that follow the sun during the day.

GHI-POA
GHI & POA in various locations in BC Canada

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Other performance factors

  1. GHI to POA ratio
  2. Far shading
  3. Inter-shading
  4. IAM
  5. Soiling
  6. Snow
  7. Low irradiance
  8. Temperature
  9. Module quality
  10. Mismatch
  11. Inverter efficiency
  12. Power clipping
  13. Wiring
  14. Transformer
  15. Dispatch
  16. PID
  17. Light soaking
  18. Aging