The effect of soiling on PV systems impacts negatively the energy production. This phenomenon is highly dependant on the environmental context and conditions of operation. Indeed dirt, dust, and other air contaminants are site-specific and their accumulation on PV modules depends on the installation configuration. This study, conducted in Santiago de Chile over a period of two and half years, focuses on analyzing power production and soiling losses of three photovoltaic technologies, monocrystalline, polycrystalline, and thin-film Si. We present the method used for analyzing time series of production data, and demonstrate the seasonality of soiling rates in Santiago city. Indeed, soiling rate values for winter season, where contamination is high, are three times higher than values for summer season, where contamination is low. A yearly trend of soiling is determined and used to study the link between economical parameters and cleaning pattern applied. The results allow us to define an optimal cleaning period depending on the period of the year.