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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. 