In the present study, the performance of mono-crystalline silicon type PV modules has been investigated theoretically at different tilt angles and orientations. The annual performance was predicted using a Fortran computer subprogram constructed based on the analysis and connected to the TRNSYS simulation program. The subprogram was experimentally verified at different meteorological conditions and design parameters in a previous study by the authors based on the meteorological conditions of Cairo (Egypt). It is found that the optimum value of yearly maximum output energy (i.e. the yearly output energy at the maximum power point) can be obtained from PV modules oriented facing south with a tilt angle in the range of 20-30degrees. Besides, the yearly maximum output energy of the PV modules mounted at different tilt angles and orientations is obtained as a fraction of its optimum value at the optimum tilt angle and orientation. (C) 2003 Elsevier Ltd. All rights reserved.