Pollution represents a major issue, and so does the ability to utilize, when available, renewable energy sources instead of traditional ones. If, on the one hand, it is possible to utilize renewable energy sources in many contexts, on the other hand they are not exploited because of the high cost of the initial investment needed for the installation of these systems, above all when domestic usage is taken into account. This paper proposes a quantitative approach able to forecast the profitability of the introduction of domestic solar thermal systems operating in parallel with the most common systems for heating domestic sanitary water. The approach is developed firstly by analyzing the most common system for heating sanitary water from both the engineering and economic point of view. At the same time the technical-economic solutions related to the most commercialized solar heating systems, and their compatibility with the most common traditional heating systems are studied. This is carried out by using a differential economic analysis of different possible scenarios in which different matches between traditional and solar heating systems are shown, and their profitability is assessed as a function of the power installed. (C) 2007 Elsevier Ltd. All rights reserved.