The amount of sunlight incident on a PV module determines how much electrical energy it produces. Since the position of the sun is constantly changing as it moves across the sky, the angle of incidence (AOI), the angle between the sun vector and the surface vector, is also constantly changing. Energy production is maximized when the PV module is pointed at the sun (using tracking systems). The increase in energy production has to be compared against the additional cost of moving PV modules to follow the sun.
The amount of sunlight reaching the earth is almost constant, around 1360 W/m2, just above the atmosphere. Atmospheric conditions like, particulates in the air, clouds, water content, and other weather conditions determine how much sunlight reaches the surface and how much gets reflected back to space. Shading from natural and man-made sources also affects incident radiation. Solar radiation incident on a surface is measured using a variety of sensors. Most common sensors measure either the light energy or heat energy of incident sunlight. Solar radiation can also be estimated by satellite measurements of cloud cover and other measurements.
Sunlight reaching a surface has two components: direct incident radiation and diffused radiation. Diffused radiation is a measure of ambient light and is due to reflections from clouds, ground, and other sources. Global horizontal irradiation (GHI) is the total irradiation incident on a horizontal surface. Plane of Array (POA) irradiation is the total irradiation on any tilted surface. Typically POA is measured co-planar with the PV module. Direct Normal Irradiation (DNI) is the radiation directly incident on a surface. DNI is typically used for two-axis tracking concentrating PV systems, while GHI and POA are used for fixed and tracking flat-plate PV systems.
Incident solar radiation measurements help to quantify the increase in energy production due to tracking. These measurements also help to design, operate, and maintain the PV system. Historical solar resource data is available for various locations from multiple sources. In the US, NREL maintains a National Solar Radiation Database (https://mapsbeta.nrel.gov/nsrdb-data-viewer/).