The width and thickness of the fins is optimized by minimizing the cost per unit useful heat flux. The proposed procedure allows computation of the necessary collection surface area. A rather involved, but still simple, flat plate solar collector model is used in the calculations. Model implementation requires a specific geographical location with a detailed meteorological data base available. Both fins of uniform and variable thickness were considered. In the case of fins with uniform thickness, the optimum distance between tube centres is decreased by increasing the operation temperature, while the optimum fin thickness is relatively the same, whatever the operation temperature and meteorological factors. The optimized width of the collection surface decreases when the operation temperature increases. The best economical performance is obtained in the case of fins with optimized space variable thickness. Optimal control techniques are used in this case. The optimum fin cross section is very close to an isosceles triangle. The fin width is shorter and the seasonal influence is weaker at lower operation temperatures. Fin width and thickness at the base depend on season. The optimum distance between the tubes is increased by increasing the inlet fluid temperature, and it is larger in the cold season than in the warm season. (c) 2005 Elsevier Ltd. All rights reserved.