Solar to thermal energy conversion was studied in order to optimize the process in a flat-plate solar collector. Two generalized relationships were used: one based on the optimum temperature of the working fluid and the other one based on the optimum path flow length. These parameters were obtained previously by means of the maximization of the exergy flow number. These optimal parameters are related to the finite conditions of operation and are considered for finite size systems, including environmental conditions variations and the irreversibilities due to pressure drop of the working fluid in the solar devices. The design procedure was applied to determine the collection surface distribution for different arrangements, in order to reach a heating load at fixed operation conditions given by the Stanton number, friction factor and collector efficiency factor. (C) 2004 Elsevier Ltd. All rights reserved.