Solar thermal power plants will provide a major share of the renewable energy sources needed in the future. STEPS, an evaluation system for solar thermal power stations, was designed to calculate the performance of such power stations as a function of direct solar radiation, geographical conditions (land slope, land cover, distance from cooling water resources, etc.), infrastructure (pipelines, electricity grids, streets etc.) and the configuration and performance of a selected solar thermal power plant concept. A cloud index derived from METEOSAT satellite images is used to calculate the direct solar radiation resource. A geographic information system (GIS) is used to process all the parameters for site assessment. In order to demonstrate the concept, an analysis of Northern Africa was performed with STEPS providing a ranking of sites with respect to the potential and cost of solar thermal electricity for a particular power plant configuration. Results were obtained with high spatial and temporal resolution. (C) 2001 Elsevier Science Ltd. All rights reserved.