In this study, the solar energy gain on vertical surfaces for heating and cooling systems in big cities, such as Istanbul, Ankara and Izmir of Turkey, has been estimated for different orientations using hourly solar radiation measurements. During summer, the monthly mean solar energy potential on vertical surfaces comes from the east and west directions more than other orientations in all the cities. However, during winter, it comes from the south, and the highest solar energy is received in Izmir, where the average annual global radiation attains its maximum. Additionally, north oriented surfaces produce between 65% and 75% lower total solar energy than other orientations in all the cities. (C) 2002 Elsevier Science Ltd. All rights reserved.