The performance of the photovoltaic power plant of Sourdun in France is studied for a period of seven years using the IEC 61724 standard. The 4.5 MWp photovoltaic installation is located in a warm temperate climate area and includes polycrystalline photovoltaic panels. The analysis of the collected data from the monitoring system reveals an average degradation rate of the power plant of 0.209%/year. The found degradation rate is in agreement with other values obtained for different power plants using the same method and having close characteristics. The degradation value is determined from the slope of the annual change in the performance ratio corrected regarding the temperature. The yearly performance ratio varying between 84.12% and 90.54% is higher in spring and lower in the summer with an average value of 87.18%. The average yearly evolutions of the reference yield and of the final yield are 3.92 h and 3.33 h respectively.