The actual amount of sunlight falling on a specific geographical location is known as insolation—or "incident solar radiation." 

When sunlight reaches the Earth, it is distributed unevenly in different regions. As you would expect, the areas near the Equator receive more solar radiation than anywhere else on the Earth. 

Sunlight varies with the seasons, as the rotational axis of the Earth shifts to lengthen and shorten days with the changing seasons. 

Other factors affect he quantity of sunlight reaching any region such as 
1. The time of day.
2. The air pollution in that region.
3. The climate (especially the cloud cover, which scatters the sun's rays).

The above factors all affect the amount of solar energy that is available to PV systems.

Insolation values for a specific site are sometimes hard to obtain because

• The weather stations that measure solar radiation components are located far apart.

• The information most generally available is the average daily total—or global—radiation on a horizontal surface.

How Solar PV Works 
How Solar Thermal Works