BACKGROUND AND OBJECTIVES: Solar panels are always installed at an angle in which receive the maximum amount of energy. Small and even 1-degree changes in the angle of the installed panel have a significant impact on the annual energy received. The objective of this study was to study the evaluation of change in the sun's radiant energy on the surface perpendicular to the radiation during the day. METHODS: Calculations of changes in the intensity of radiation on the surface of the panel are generally performed by assuming that the intensity of solar radiation is constant on the surface perpendicular to the radiation during the day (choosing the solar constant) and multiplying it by the cosine of the azimuth angle (which varies during the day). Since the sun's rays travel different lengths in the atmosphere at different times of the day, the intensity of the sun's radiation on the surface perpendicular to the radiation varies throughout the day. In this study, the effect of daily changes in the intensity of solar radiation on the surface perpendicular to the radiation, on the optimal angle of the solar panel has been investigated. FINDING: The results showed that the daily optimal angle difference reported in this study compared to previous studies is more than 5 degrees in some cases. Also, installing the panel under the optimal daily angle (for day number 100) and the correct yearly angle resulted in receiving 128.56 kilowatts per square meter and 2.977 megawatts per square meter more energy, respectively. CONCLUSION: According to the results of this research, the annual optimal angle for a geographic latitude of 30 degrees, taking into account the changes in solar radiation energy on the surface perpendicular to the radiation, is 26 degrees, which is 4 degrees different from the geographic latitude. Also, the results show that if the panel is installed at an angle of 30 degrees, the energy received annually is 16.122 megawatts per square meter less than if the pan
