This work is performed to investigate the effect of using different sun tracking mechanisms on the flat plate photovoltaic system performances and the main parameters affecting the amount of their electrical energy output as well as those affecting their gains compared to the traditional fixed photovoltaic systems. To this end, five configurations of sun tracking systems and two traditional fixed panels have been considered. The sun tracking systems effect on the PV system performances is improved by using the hourly data collected over 18 days for different seasonal sky conditions. The daily cumulative electrical energy produced by the different systems have been quantified separately for each sky state and the corresponding electrical gains have then been compared to those experienced with two traditional fixed photovoltaic systems. It is found that for a completely clear day, the highest obtained gains are those related to the two-axis sun tracker systems, which decrease gradually from the inclined to the vertical rotating axis when the same optimum slope is applied and from the seasonal to the yearly optimum slope if the same rotating axis is considered. On the other hand, for the partially clear days, the gain amounts are mainly dependant on the clearness index and on the seasonal variation of day length values. For a completely cloudy day, the results show that all considered systems produced closely the same electrical energy and the horizontal position of the photovoltaic panel presented the best performance. (C) 2010 Elsevier Ltd. All rights reserved.