The position of the sun relative to an observer on the surface of the Earth is an important input needed to model PV system performance.  The convention used to describe solar position includes:

• Zenith angle ($\theta&space;_{Z}$)
• Azimuth angle ($\theta&space;_{A}$)
• Solar elevation angle ($\theta&space;_{el}$) is equal to $90^{\circ}&space;-&space;\theta_{Z}$

The figure below shows how these angles are defined.