Nowadays, solar energy and photovoltaic panels are taken under serious consideration. Many studies have been conducted on pv modules deficiency. In addition to the characteristics of the panels, the climate and natural condition also have a great impact on the performance of these systems. In order to recognize the impact of dust accumulation on PV performance in Tehran, Iran, a 70-day experiment, started on May 9th, 2017, was developed. Equipment at the Renewable Laboratory of Shahid Beheshty University was utilized to obtain the experimental data. Power reduction as well as delivered energy, and its reduction during the test period were measured. For more accurate and general conclusions, the curves were also plotted versus dust surface density. The results indicate that after 70 days without raining, 6.0986 (g/m(2)) dust was accumulated on the surface, which caused 21.47 (%) reduction in the power output. The total reduction in delivered energy was also 289 kWh for each 4.845 kW power capacity. The reduction of this amount of clean energy is equivalent to 3 hectares of forest absorbing carbon (32.7 tons) per MW capacity.