Photovoltaic arrays are known to suffer power efficiency losses over time due to accumulation of natural dirt and dust. The importance of cleaning in order to maintain efficiencies and the significance of natural cleaning by rainfall have not been widely studied in different climates. Monocrystalline silicon photovoltaic panels located in Portland, Oregon, were evaluated for the effects of natural soiling on power output and correlated with efficiencies after manual cleaning or natural rainfall. The masses of particulates on each panel were measured when cleaning the panels, and the effects of the manual cleaning and natural cleaning by rainfall were compared. In order to distinguish possible causes for the losses in efficiency, thermal effects of soiling were also studied. During a 17-day rain-free period in July and Aug. 2011, natural particulate deposition was measured at 0.85 g/m2, which led to a power output about 4% lower than a nominally identical clean panel. A single natural rainfall event was sufficient to clean the panel to a level that restored power output to within 1% of the manually cleaned panel. Natural particulate deposition at that level did not detectably affect panel temperature, suggesting that the power losses were due to optical scattering effects rather than temperature effects. Artificially managed temperature adjustments did significantly affect power output, consistent with the expected temperature effects for monocrystalline silicon. Given the effectiveness of natural rainfall in cleaning the panels, appropriate protocols for maintaining optimum efficiencies can be determined for different climate situations.