Methods applied on efficient planning of ground-based monitoring networks of surface solar irradiance could provide valuable scientific results and be useful for accurate monitoring and efficient planning of solar energy applications. Based on the dominance of cloud effect on solar irradiance and the advantage of the high spatial resolution of a geostationary satellite, a novel method is presented for optimizing the location of measuring sites for the newly built Hellenic Network of Solar Energy (www.helionet.gr). The k-means algorithm is used for cluster analysis and the validation of the clustering method reveals that the variability of surface solar irradiance due to cloudiness over Greece could be sufficiently monitored with the establishment of 22 ground-based instruments. The spatial representativeness of the proposed sites is also assessed. The proposed number of stations could be considered as the basis to build the climatology of surface solar irradiance over Greece. (C) 2013 Elsevier Ltd. All rights reserved.