## 1 Introduction  
The study of the reliability and long-term durability of photovoltaic (PV) modules has a great impact on the energy forecast, the economics of the power plants, and the planning of the operation and maintenance activities of the PV systems [1, 2, 3].  
Different performance analyses have been carried out with different technologies, plant topology, and placements [4, 5, 6, 7, 8, 9], wherein the results indicate the PV modules have provided electric power for more than 20 years. However, errors determining degradation rates can increase financial risks in the PV sector [10].  
In outdoor conditions, to determine how long a PV module or system will last is a challenge because they are influenced by several issues, such as the local climate, technology, materials, and manufacturing, as much as the installation conditions themselves [11].  
After more than 40 years of field testing, in different world locations, degradation rate distribution of the PV power plants have reported around 0.5%/year [10, 12, 13]. Therefore, this value has been also adopted to consider long-term crystalline silicon PV module degradation in financial models [14]. Lately, annual degradation rates have been estimated on PV modules, using modern cell technologies as well as encapsulating techniques,which reduced the average degradation rate at 0.4%/year [15], wherein values lower than 0.27%/year can be achieved for crystalline silicon technologies, operating in utility-scale PV facilities subjected to proper operation and maintenance practices [16].  
To perform an analysis of the durability, several parameters are required to evaluate the degradation, such as the I-V curve evolution [15], color change of the PV panels [17], or the temperature coefficient variation [18], among other approaches. However, the analysis of the performance ratio (PR) is one of the most widely used methods [19; 20; 21; 22; 23; 24], mainly due to its accuracy and non-dependence of external factors.  
Since a single set of degradation measurements based on one measurement are not representative of the population to estimate true degradation of a PV installation, repeated measures across several groups are necessary. The analysis of the operation data and performance through linear mixed effects models (LEMs) is a suitable tool for analyzing longitudinal data to explain the degradations in PV modules/systems [25]. Even more, it is reasonable to assume a linear degradation model, although some publications use an exponential degradation model [26] or classical series decomposition [20]; it is shown that for a typical starting degradation rate, these models do not differ significantly during the first 10-15 years [27].  
In this paper, the performance of 8 years of a 13 MW installation located in the South of Tenerife (Canary Islands, Spain) was analyzed. LEM approaches were used to evaluate the evolution of the PR of the PV production units in which this facility is articulated, and its degradation rate, considering the effects of technology, manufacturer, and nominal power of photovoltaic modules used during its installation, as well as the distribution of said PV units within the facility. The concept of degradation rate is used in this work, wherein robust estimation of the true degradation in a specific environment was evaluated [28].