Estimation of Solar Energy Using Different Empirical Models at Okhaldhunga, Nepal

Accurate estimation of global solar radiation (GSR) is essential for the effective design, sizing, and performance evaluation of solar energy systems. However, such data is often lacking in many regions of Nepal due to the high cost of measurement. This study investigates the performance of various empirical models to estimate daily GSR in the eastern mid-hill region of Okhaldhunga (27.3081°N, 86.5042°E, 1731 m) using data from 2021 to 2023. Models based on sunshine duration, temperature, and relative humidity were evaluated using regression analysis and statistical indicators such as Root Mean Square Error (RMSE), Mean Bias Error (MBE), Mean Percentage Error (MPE), and the coefficient of determination (R²). Among the tested models, Model G—using maximum temperature and relative humidity—performed best, with the lowest RMSE and the highest R² value (0.6998). The empirical constants for Model G were determined as: a = 0.1056, b = 0.3136, c = 0.0186, and d = –0.0024. These constants can be effectively used to predict GSR in other regions of Nepal with similar geographic and climatic conditions.