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The optical loss caused by the accumulation of dirt and other contaminants on solar collectors is the third most important meteorological input, after insolation and air temperature, that determines energy yield in a photovoltaic power plant. First Solar is monitoring the impact of dust accumulation on solar panels using a methodology similar to that proposed by Ryan and colleagues in a long-term study in Eugene, OR. The method seeks to be a practical and automated technique for measuring the soiling rate, foregoing complex equipment such as I(V) curve tracers, and eliminating other performance-influencing factors. The test setup includes several First Solar plane-of-array modules that are allowed to accumulate natural contaminants, and several controls held clean by regular washing. This study reveals clear region-specific soiling trends as well as insight into the amount of rainfall required for full recovery of module performance. Soiling rates of up to 11.5% per month are observed in heavy agricultural areas. As little as 0.5 mm of rainfall is sufficient to completely clean a dirty frameless module in regions with lighter soiling rates.