A simple calculation method to calculate the mean hourly global, direct and diffuse illuminance on horizontal and vertical surfaces for all sky, clear sky, intermediate and overcast sky conditions, initially developed for Arcavacata di Rende (Italy), was compared with experimental data obtained at Geneva (Switzerland), Vaulx-en-Velin (France), Bratislava (Slovakia) and Osaka (Japan). The method is based on the use of constant luminous efficacies, adopted also for the vertical surfaces. 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 B.V. All rights reserved.