It is common knowledge that the solar-thermal energy efficiency increases without extremum points with the flow rate, and similarly the fluid outlet temperature increases with the collecting area. The absence of maximum points for the functions eta(en) = f(m) and T-f,T-o = f(Ac) has created difficulties in the design of flat solar collectors. The exergy efficiency of a flat-plate solar collector, eta(ex) = f(m,AC) presents points of local maxima and a point of global maximum. In order to eliminate the drawbacks arising from the random variation of T-f.i. the paper proposes an exergy analysis of a flat-plate solar collector based on the assumption that T-f,T-i = T-e = const. (T-e - environment temperature). The method has proven valuable in the design of solar collectors for the specific climatic and insulation conditions of a certain region. (C) 2004 Elsevier Ltd. All rights reserved.