A simple calculation method to calculate the mean hourly diffuse illuminance on vertical surfaces for all sky, clear sky, intermediate and overcast sky conditions, developed in Arcavacata di Rende (Italy), was compared with experimental data obtained at Osaka (Japan), Vaulx-en-Velin (France) and Geneva (Switzerland). In spite of its simplicity, the method furnishes reasonably good predictions, in comparison with a more complex reference calculation method and can be proposed as a simplified tool for design purposes. (c) 2008 Elsevier Ltd. All rights reserved.