One of the major reasons for the poor performance of PV systems in the MENA region is dust accumulation. Unlike concentrated solar power (CSP), poly-crystalline and single-crystalline PV panels have shown power reduction in excess of 60% due to dust particles. This paper presents a study on the impact of dust on power generation by the 500kWp PV system installed at the University of Bahrain. The study is based on eight PV panels out of an array of 2,088 PV panels forming the 500kW system. The eight panels under study have not been cleaned since October 1, 2013. It will be shown that the density of the accumulated dust ranged from 5-12g/m2 with an average PV power drop to 40% of the maximum available. The collected dust was analyzed revealing that the main elemental contents were Si (~ 15%) and Ca (~ 15%) in addition to Al (~ 6%), Mg (~ 5%), and Fe (~ 5%). Finally, two important empirical equations were established that allowed the estimation of the loss in light transmissivity (in%) due to dust accumulation (g/m2) on the PV panels, as well as a drop in PV power output as a result of dust density.