In the present work, the reliability of the "INPUT-OUTPUT" (I/O) method, as a tool for the description of the energetic performance and the prediction of long-term yield of Large Solar Thermal Systems (LSTS) is being investigated. The efficacy of the method over a wide range of conditions, by taking into account the possible repercussions from non-typical operating conditions of the solar system, is also evaluated. More specifically, the repeatability of the method and the determination of the main prerequisites (such as range of daily solar radiation, number of measurements, seasonal distribution, initial energy level in the tank etc.) ensuring the similarity of the physical processes are being investigated. Finally, as an additional study of the robustness of the method, an estimation of the discrepancies in the predictions of the model, when these are compared against experimental data not used for the determination of the coefficients of the characteristic equation is carried out. (C) 2012 Elsevier Ltd. All rights reserved.