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# Glossary

### Probability of Exceedance

In terms of solar resource data, probability of exceedance refers to the probability that the total annual irradiance will exceed a given amount. Probability of exceedance values are often reported as “PXX” where the “XX” represents the probability value. For example, a P90 irradiance total of 1,535 kWh/m2/year means there is a 90% chance that the irradiance total for the year will exceed 1,535.

Probability of exceedance values are helpful in assessing multiple PV energy yield scenarios. If you would like to learn more about SolarAnywhere probability of exceedance data, visit probability of exceedance.

### Solar Zenith Angle

The zenith angle (αzenith) is used in determining the position of the sun in relation to a specific location on earth. It’s used to calculate the amount of irradiance reaching that location at a given time of day. It is the angle between a line that is normal to a horizontal surface on the earth and a line drawn directly to the sun from the horizontal surface. ### Angle of Incidence

The angle of incidence (αzenith) is the angle between a line that is normal to the surface of the PV module and a line drawn directly to the sun from the module surface. If the module surface is horizontal to the earth, then the angle of incidence and the solar zenith angle will be the same. Plane of Array Irradiance (POAI) refers to the solar irradiance reaching the plane of the PV module. POAI is calculated using a transposition model that considers the PV module array azimuth and tilt or tracking to calculate the irradiance reaching the module surface. SolarAnywhere energy models use the Perez transposition model, which also considers the effects of row-to-row shading.

### Aerosol Optical Depth (AOD)

This is a quantification of the aerosols, such as dust, haze, ash, sea salt, etc., distributed between the surface of the earth and the top of the atmosphere. Aerosol optical depth is an important consideration in satellite-based solar irradiance modeling and is used in calculating clear sky irradiance. It can help us measure the impacts of things such as coal-fired power-plant closures or wildfires on the solar resource available for PV energy production.

Recent SolarAnywhere versions utilize publicly available aerosol optical depth data from the Merra-2 reanalysis. Visit release notes to see the AOD input changes accompanying SolarAnywhere version releases. The validation for this AOD source is available here.

### Isotropic

Isotropic refers to a property or characteristic of a substance, material, or system that remains the same in all directions. Within the solar industry, the term “isotropic” typically describes a component of solar radiation that is uniformly distributed in all directions. It implies that the solar energy reaching a particular surface is the same, regardless of the orientation from which it arrives.

When modeling solar energy systems, the diffuse irradiance may be assumed to be isotropic, meaning that the scattered component of solar irradiance is evenly distributed as it reaches a surface. This assumption simplifies PV modeling calculations by treating the diffuse irradiance as uniform and isotropic, enabling more efficient assessment of PV system performance.