For the last decade, the energy supplies has become a very preoccupying problem, not only because of the increasing difficulties bound to oil production, but also because it is necessary today to admit that at the scale of our planet the energy resources, fossil-fuel or other, are limited. The solar energy is the only outside energy whose contribution is permanent to the global scale. However, real progress of solar field depends widely on its economic cost. In this context, this investigation focuses on a techno-economic study of a solar power unit for heating sanitary water at low enthalpy. A software program, based on a mathematical model, was elaborated, in order to identify criteria of an optimum use. These criteria take into account, in particular, the technical models, the recent technological innovations and an accurate analysis of the commercial balance (investment costs, actualised values of energy savings, profitability ratio, etc.). The solar installation production is determined by climate conditions, energy demand, collector area, material type and load capacity A sensitivity analysis is carried out to determine the influence of these parameters, and many others, on the yearly capacity. This paper presents the results of a program concerning water heating in a partial solar central power station. The study aims, on the one hand, to help the fitters and the industrials in the calculation and the design of such plants, and on the other hand, the economists will be allowed to compute the different financial and economic parameters in deal with the solar systems.