© 2019 Yasuyuki Ota et al. The radiation received by solar cells within photovoltaic modules is lower than that arriving at the module surface. One of the causes of this energy loss is soiling of the module surface. Therefore, the influence of dust adhesion on photovoltaic modules must be studied. In this study, we prepared two tracker systems: a new system and a typical system. During the night, the former can switch to a downward-facing standby state, while the latter assumes an upward-facing standby state. The soiling on the polymethylmethacrylate and glass set on the tracker systems with both standby states was evaluated for 20 months in Miyazaki, Japan. By adopting the tracker system with the downward-facing standby state, a direct transmittance that was more-than-5% higher than before was consistently obtained at 500 nm in both cases with polymethylmethacrylate and glass.