In the present paper a genetic optimization (GO) has been carried out on an office room with a south facing window in order to design an optimal fixed shading device. Two different glazing systems have been taken into account, one standard double glass and an high performance glazing system specifically designed to prevent high sun loads. The shading device is a flat panel positioned parallel to the window and inclined by its horizontal axis. The device shades the window from direct sun penetration reducing the cooling loads in summer, but also affecting daylight and heat loads in winter limiting the sun gains, therefore the impact on the overall building energy consumption is investigated. A genetic optimization has been performed for identifying a possible geometry with the lower energy impact. Lighting loads, computed by the DAYSIM code, have been considered as inputs for the code ESP-r which drives the energy computation. The results demonstrate that electrical energy absorbed by the lighting system has to be always taken into account in designing energy efficient shading devices. (C) 2014 Elsevier B.V. All rights reserved.