The efficiency of a flat plate collector is estimated. The transfer of heat energy through its absorber plate considered as its main part of structure is studied by solving the heat diffusion equation using the Fourier transform technique. The temperature of the working fluid is also obtained considering the heat balance equation and a simple suggested model for the considered collector. An expression useful for practical purposes to predict the hourly global solar irradiance on horizontal surface is given. It shows good fitting with corresponding experimental data recorded in Makkah, Jeddah and Hong Kong. The obtained formula is used in the theoretical analysis of the present heating problem. The trial is promising and shows that the dependence of the efficiency on the revealed factors is nonlinear. (C) 2002 Elsevier Science Ltd. All rights reserved.