In analyzing the solar energy systems, the first step is the accurate measurements/estimations of the input in the long term: solar irradiation incident on the panels. The measurements are lacking in many places so the estimation procedures should be developed using the measured data of the locations having similar climates and close in latitudes. In most life cycle assessments, monthly averages of daily values are utilized in the form of typical meteorological year or simply with the averages of a year or for a number of consecutive years. Although the beam component of the solar irradiation is usually higher than total of all diffuse components except in highly cloudy and overcast sky conditions, estimation of diffuse components is vital especially to determine the solar irradiation on tilted solar panels. In this study, with the final aim of developing the best procedures to estimate diffuse solar radiation component for all the locations of Turkey, we carried out a preliminary comparison and reached a best procedure of estimation for two locations in Central Anatolia. Our procedure is to compare a model based on a physical formalism developed before with four methods of estimation appeared in the literature, together with the data obtained by Meteonorm Software. As a result, our model has given one of the two best results together with Meteonorm Software when compared to measured values. It is in fact the best one according to the RMSE value.