The development and deployment of photovoltaic power plants is still growing all over the world with the significant decrease of costs and by reducing the energy dependence to fossil fuels especially in some countries in the MENA region. Besides, to the abundant solar radiation in this region, the semi-arid and arid climate promote harsh weather conditions as high dust concentration which cause a significant degradation on the power production and performance of the PV systems. Dust analysis and the transmittance soiling loss with regard to the dust surface density under real conditions for glass samples exposed on fixed tilt with 32 degrees and tracking system with horizontally stow position during the night were investigated in the mid-south of Morocco for a period of one year from June 29th, 2018 until June 28th, 2019. The results indicate that the transmittance drop is generally significant for the fixed PV system and is strictly dependent on climatic conditions during the investigation period. The accumulation rate of dust decrease the optical transmittance over 28% after 30 days of exposure, which reduce the electrical energy of the PV system by 4.4%. In addition, by increasing dust surface density from 0 g/m2 to 1.1 g/m2, the electrical power reduces by 14.2% for the fixed PV system. On the other hand, the results have demonstrated that for a PV module exposed for one year at the same climate conditions, the electrical performances have decreased by 27% after 8 months of exposure without any cleaning event.