## 1 Introduction  
As the energy mix continues to change around the world, fossil fuels are being replaced by renewable energy sources. Renewable energy, unlike fossil fuels, never run out and no greenhouse gasses or other pollutants are created during the process of energy generation. With the rapid growth of renewable energy over the last 10 years, solar and wind power are now the cheapest sources of electricity in many parts of the world [1]. In 2021, global electricity power generation capacity from solar sources achieved 849 GW [2]. According to statistics published by Fraunhofer [3], the solar energy of electricity production in Germany reached 9.1% in 2021. Furthermore, Germany is planning to fully implement a 100% renewable energy plan and guarantee photovoltaic growth up to 215 GW until 2030 [4]. Thus, photovoltaic power has a very high potential for future power development. In recent years, as the installed capacity of photovoltaic (PV) systems continues to increase, their performance and energy yield have received increasing attention from investors and researchers. In addition to the inherent limitations of module materials, there are many important and uncertain factors that have an impact on the power-generation efficiency of PV systems in complex and changeable climate conditions [5] such as:  
* Faults of PV modules, e.g., manufacturing defects, cracks of solar cells, potential induced degradation (PID), defective bypass diodes, natural aging, etc. ;* Shadow, soiling, irradiation deviation, snow coverage;
* Efficiency decreasing by temperature rise;
* Series-parallel unbalance and direct current (DC) losses;
* Inverter losses, mismatch to maximum power point (MPP). If the presence and interaction of these effects are undetected or non-monitored of a prolonged time, these can lead to reversible or irreversible damage that reduces the output power of the PV system and may even cause safety issues [6]. Therefore, performance assessment of PV systems is essential to provide early warning of faults to maintain uptime through proper monitoring. The most common approach is the framework of International Electrotechnical Commission IEC 61724 standard, which forms the base for evaluation of field performance of PV plants in 1998 [7]. It includes few parameters of the reference yield (\(Y_{r}\)), array yield (\(Y_{a}\)), final yield (\(Y_{f}\)), the performance ratio (\(PR\)), system losses (\(L_{S}\)) and array capture losses (\(L_{C}\)). These quantities express the system-level performance in relation to energy generation, solar resources and the overall effect of PV system losses [8; 9; 10]. Since then, many studies have investigated, evaluated and analyzed PV systems with the standard IEC 61724 in different locations. In France, [PERSON] et al. [11] studied the performance of the 4.5 MWp photovoltaic power plant with 7 years operation, which is located in a warm and temperate climate area. The annual performance ratios are between 84.12% and 90.54% (yearly average 87.18%), with higher rates in the spring and lower rates in the summer. In the literature [12], the performance and degradation evaluation of a 1 MWp utility-scale photovoltaic system located in the tropical semiarid climate of India was observed through 4 years of monitored data. The reference yield, final yield, system efficiency, capacity factor, and performance ratio were 6.23 h/day, 4.64 h/day, 11%, 19.33% and 74.73%, respectively. After 50 months of operation, the degradation rates were assessed through the linear least squares regression (LLS), the classical seasonal decomposition (CSD), the Holt-Winters seasonal model (HW), and the seasonal and trend decomposition using loess (STL) to be 0.27%/year, 0.32%/year, 0.50%/year and 0.27%/year respectively. As we know, the weather conditions including irradiance, ambient temperature, humidity, wind speed and direction have great effect on the efficiency of a PV system, which is also affected by other environmental factors, such as dirt and dust.