Comparative Analysis of Long-Term Solar Resource Databases for Global Horizontal Irradiance Prediction
F. Sousa(1), J. Mendonça(2), R. Pilão(1) and P. Pinto(3) 1. CIETI, ISEP, Instituto Politécnico do Porto, Porto (Portugal) 2. SIIS, Instituto Politécnico do Porto, Porto (Portugal) 3 MEGAJOULE S.A. Maia (Portugal) 2026-02-15

Abstract

The short measurement periods of local solarresource measurement campaigns limit the representativeness of these measurements in long-term energy production estimates for solar projects. This study aimed to characterize the main long-term solar resource databases and correlate them with data provided by meteorological stations, with the purpose of identifying the database that best predicts solar radiation at specific locations. Seven long-term databases were used (PVGIS, SOLARGIS, SOLCAST, NASA POWER, NSRDB, HELIOCLIM, ERA5). Measured data from meteorological stations at various locations in Portugal, Saudi Arabia, and Brazil were correlated with the long-term databases available for each location. Different methodologies for evaluating these correlations were tested, including R², MSE, MBE, and MAPE. The analyses revealed that the SOLCAST and SOLARGIS databases highlighted in predicting global horizontal irradiance (GHI) in the Arabia region. In Portugal, the NSRDB was the most accurate in predicting GHI, while in Brazil, SOLCAST showed the highest accuracy in forecasting GHI.

Key words: Solar resource, local measurements, long-term databases, error metrics

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