A photovoltaic system connected to the grid is continuously monitored in order to evaluate the performance and the efficiency of conversion. The monitoring system detects abnormal operations and power fluctuation in order to execute appropriate tasks. It gives the possibility to optimize control laws and to implement interconnection strategies. Moreover, it is used to evaluate the cost effectiveness of solar energy penetration.         Therefore, a database is formed of measured values on parameters that affect the system operation. It is continuously updated in order to improve accuracy of results and to take real time decisions. This study suggests a presentation method of these parameters’ values in order to directly deduce the performance and detect abnormal operations. Measurements are performed in the Centre of Renewable Energy Sources and Saving (CRESS) in Pikermi, Athena, Greece.