A thermoeconomic analysis based on the Second Law is presented. It combines the annualized total cost (investment, installation and operating costs) with entropy generation during the collection and thermal use of solar energy. This thermoeconomic model is developed to determine the annualized total cost for air heating in a solar air heater by means of dimensionless parameters like the Entropy Generation Number and the Mass Flow Number. With these dimensionless groups, a set of relations between optimum operational variables, physical characteristics and properties of construction materials of the heater, are established. The function incorporates the cost and quality of the product, the cost by solar-to-thermal energy conversion, and also considers the thermal energy quality through the Second Law Efficiency.