An exhaustive evaluation of the performance of decomposition models to estimate direct and diffuse components from the global horizontal solar irradiance has been carried out in this work. The main objective of this work has been to compare the model performance for two different time bases, hourly and monthly average hourly bases. An extensive database of horizontal solar irradiance from nine locations in Spain was used for the study. The data span through January 1980 to December 2012 of hourly solar irradiance for the nine locations and thus indicate a cumulative year sum of 132 years. This study first investigated the decomposition of the hourly horizontal irradiance into hourly direct and diffuse components using six decomposition models widely referenced in the bibliography. In the hourly decomposition investigation, it was observed that there are no significant differences between the six models for each specific location. Nevertheless, the performance of each of the models was strongly dependent on cloudiness conditions and the solar altitude at the location which is associated with the climatic condition of each site. Further investigations using the six decomposition models were conducted to estimate monthly average hourly values of direct and diffuse components of the solar irradiance with proper assessment of the different models' performance at various locations. Based on the results of the investigations which present no significant differences in the performance of the different models, an extremely simple algorithm was developed to estimate monthly average hourly values of direct and diffuse solar irradiance, which reduces the statistical errors in all locations investigated.
