This paper presents losses comparison and analysis results for different types of fault modes of
grid-connected photovoltaic system generated for long-term operation. The approach of losses
comparison and analysis by faults is to identify relationship between measured and estimated
values of five loss factors which are quantified from irradiance to system output power. This
paper presents the symptom results for faults such as snow, shading, sensor defect, blackout,
soiling and so on from three years or more monitored data. These results will indicate that it is
useful to develop fault detection and diagnosis tool to enhance capacity factor and save
operation and maintenance cost of grid-connected photovoltaic system in the field.