Estimation system of in-plane irradiance by regression functions of in-plane/horizontal irradiance ratio vs. time

Photovoltaic (PV) system performance evaluations are required at the system deliveries to users. For the performance evaluations, it is very important to estimate the irradiance on the modules of PV system to design energy system or to check the energy output on-site. In this paper, we propose the estimation system of in-plane irradiance by regression functions from measured irradiance data. The system consists of the monthly regression functions R(t) of the minutely ratio of in-plane and global irradiance separated by clearness ratio. The data in three years for 100 residential PV sites in Japan were used to calculate and analyze the regression coefficients of these sites.