Input/output diagrams are very useful for evaluating the daily performances of solar collectors. We present a simple and accurate method to generate such diagrams for solar collection subsystems. The introduction of new dimensionless variables leads to a simplified and universal expression of I/O diagrams. The performances of the entire solar systems can then be evaluated by combining I/O diagrams to day by day simulations, which is very helpful in order to check existing installations or for design purposes. All these procedures are based on and validated by experiments, measurements and studies performed over many years.