Different types of tools, software and empirical methods to size and assess the performance of solar thermal systems are available today. A quick and easy-to-use graphical tool is proposed here. It is based on,a single diagram, called a nomograph, which embeds technical and economic information, allowing the optimisation of the size and performance of a solar thermal system starting from the main input parameters, such as the specific costs of the system and the auxiliary fuel. The optimal surfaces are calculated as a function of the fuel cost through several regressions made on data from the application of the F-Chart method for a set of system configurations. The authors have found good correlations among these two parameters for given collectors types, climates and fuel costs. Furthermore, the design information stemming from the use of the diagram considers the optimal area of the surface of solar collectors, the corresponding annual solar fraction, the annualised total cost of the solar system and the conventional systems meeting the same load, the annual savings achieved with the use of solar system and the corresponding payback time. Nomographs have been drawn for three Italian locations, representative of different South Europe climates, for a given demand of hot water and for two types of collectors. The methods and algorithms related to the construction of the diagrams are described in this paper. Additional nomographs can be produced through a standardised procedure based on Excel Macros. An analysis of the results obtained is then performed for a sample application. (C) 2012 Elsevier Ltd. All rights reserved.