The ideal inclination of tilted surfaces, used to maximize the capture of surface solar irradiance, is determined by latitude and time of the year. Although several algorithms are used for its estimation, the effect of clouds is difficult to be taken into account and causes large deviations from the "clear sky scenario". Our aim is to investigate the solar irradiance at inclined surfaces on real atmospheric conditions. A set of four pyranometers, located at the National Observatory of Athens, is used to measure the solar radiation for one year and at 1-min frequency. We use the Global Horizontal (GHI) and the Global Irradiance (Gip) reaching surfaces tilted at a 0 angle, with different orientations, in order to quantify the energy benefits of different installations. Furthermore, model calculations were used to simulate Gip at different tilt angles. GHI measurements agree within the theoretical calculations on cloudless days, receiving more irradiance than the inclined surface during summer months. However, the G1 beta reach higher values than GHI in wintertime. Model calculations for various tilt angles reveal that the optimum one is around 30 on year basis. (C) 2017 Elsevier Ltd. All rights reserved.