Conventional methods to estimate the photovoltaic (PV) power generation of an ensemble of PV systems are based on measured reference systems. However, these methods usually do not consider the module orientation in terms of azimuth and inclination angles. In reality, the orientation of PV systems can differ significantly, leading to large errors in this procedure. This paper presents an approach which improves the computation of the overall power from PV generation, based on measurements from reference systems, by including the module orientation for each system individually. The application of the methods to a dataset of 45 PV systems with five-minute resolution in Freiburg between 2010 and 2014 shows promising results. The relative root-mean-square error of the overall PV-power estimated from single reference plants improves by up to 19.7%. In addition, the performance of the methodology to estimate the global irradiance in the horizontal plane and at a target module orientation from the PV-power of a reference plant is assessed. (C) 2016 Elsevier Ltd. All rights reserved.