In this study, a theoretical and experimental investigation of a new modeling method for solar thermal processes, based on the input-output (I/O) approach is presented. This approach is characterized primarily by the consideration of the solar thermal system, irrespective of size, as a functionally integral set, the daily yield of which is expressed as a function of the total daily radiation, the average ambient temperature and the energy level of the storage tank at the beginning of the day. The study focuses first on the theoretical foundation of the method, based on the analysis of the evolution of the main energy quantities during a daily cycle, with emphasis on the energetically most important phase of the pseudo-steady state. By integrating the instantaneous energy balances over the entire duration of this phase, analytical expressions for the coefficients of the characteristic I/O equation arise, as a function of the main characteristics of the solar system. Subsequently, the experimental validation of the proposed approach is presented, based on the comparison of theoretical results with extensive measurements on two typical large-scale solar systems, one with a fully-stratified and the other with a fully-mixed thermal storage tank, respectively. (C) 2009 Elsevier Ltd. All rights reserved.