For the estimation of the expected annual energy yield and the month-by-month check of a PV system's performance, methods based on irradiation maps published by weather services, both general or dedicated to solar energy application, are in use. Examples for this type of information for Germany are the annual and monthly irradiation maps as published by the German Weather Service (DWD), the data bank of hourly irradiance data with continuous spatial coverage prepared by the University of Oldenburg, and the irradiation data products of the Climate Monitoring Satellite Application Facility (CMSAF) assembled by a consortium of European Weather Services. To assess the validity of these data sets for the aforementioned tasks, a case study is performed for the region of the German federal state of Saxony using data for the year 2005. For this region a set of ground-measured irradiance data from meteorological stations operated by the German Weather Service (DWD) is available. The comparison of hourly irradiance information is done by the monthly analysis of the bias and the RMS-error for the Oldenburg data bank of hourly values versus the ground station data. For an additional inter-comparison of the different data sources, the annual maps presenting the estimated irradiation sums for 2005 are analyzed. This gives information on data accuracy with respect to the spatial structure of the irradiation field. The assessment of the end use accuracy of the irradiation data is made by investigating a set of monthly energy yield data of grid-connected PV systems. This is done via the estimation of the energy yield using a PV system simulation applying the irradiation data generated at the University of Oldenburg. (C) 2008 Elsevier Ltd. All rights reserved.