The accumulation of dust on the solar cells panels worsens the situation and lowers the efficiency of the solar cells day by day especially in the regions known by their high rate of dust, low frequency and intensity of rain. The accumulated dust on the solar cells panel blocks the cells from the sun's rays and act as a screening effect as shown by the calculated spectral transmittance of dust which decreases the performance of the solar cells over time until the cell panels are cleaned manually or by rain. The tilt angle of the solar cell panels affects considerably the amount of accumulated dust on the surface of the panels.The study focuses on the effects of dust accumulation on photovoltaic solar panels in Jazan region. The effect is tested in outdoor measurements and it was found that regular dust accumulation reduces the solar cells efficiency by about 10% for an exposure time of 16 weeks. Moreover the tilted panel with an angle of 30° receives more dust than the panel tilted by 50° which means higher efficiency loss for the lower tilted panel. The efficiency loss is significant of large of annual monetary losses. This study can be considered as a reference to predict the level of degradation of any photovoltaic system that might be installed in the Jazan region and also to estimate the real cost include the cost of the solar panel cleaning.