Although many small-scale photovoltaic power plants employ remote sensing systems, these data are merely utilised to operation and maintenance because detail analysis is often needed to recognise plant conditions including weather and other environment information from these csv data. This paper reports a development of a simplified low-cost monitoring service especially for small-scale photovoltaic power plants to estimate damage and trouble factors from fewer monitoring data of generation and public weather information. The authors also propose “Simplified System Performance Ratio (sPR)” that can grab degradation of annual generation of photovoltaic using simple generation monitoring systems and data from neighbouring weather stations. This paper introduces the result of investigation of totally 272 power plants in Japan and relationship between various parameters and sPR’s degradation using regression analyses. The result clarifies that there is a statistically significant correlation between sPR and FIT (Feed-in Tariff) buying price as well as so-called “low-voltage divisions”, which means a group of plants that are intentionally divided into small plants less than 50 kW.