## 2 Materials and Methods  
### Description of the Photovoltaic System  
The analysis of the PR and its degradation rate was performed in SOLTEN, a 20 MW utility-scale PV facility located in the south of Tenerife, Canary Islands, in the municipality of Granadilla (see Figure 1), which was installed and operated by the Instituto Tecnologico y de Energias Renovables, S.A. (ITER). The Canary Islands are located in the trade wind zone on the southeastern side of the Azorean anticyclonic system, but also relatively near to the West Coast of Africa. As a result, the islands have a subtropical climate, characterized by hot summers and mild winters with infrequent frost. However, due to the island's orography, they are also subjected to other climates that have a local effect, called microclimates [29]. Thus, according to the Koppen climate classification [30], the climate where the PV facility is located is similar to hot arid desert, with high levels of solar irradiation, but, due to its closeness to the coast, it is also heavily exposed to a salty and humid environment. The SOLTEN 20 MW PV facility was developed on a plot of land of 217,775.80 m\({}^{2}\) between 2006 and 2008 and is made up of 200 PV generation units. Each unit has 100 kW of nominal power, occupies an area of about 820 m\({}^{2}\), and is set up at 10\({}^{\circ}\) tilt and true North-South orientation. In all units, the PV modules are clamped to light aluminum structures, totally modular and removable, being formed of pillars, girders, and beams, mounted over profiles of galvanized steel, embedded on concrete foundations (see Figure 2). The PV production units were placed in rows, leaving streets between each row of plants, both to minimize the impact of shadows and to allow their servicing. Since the plot of land has a diamond-like shape, each row accommodated a different number of plants, depending on its location. Following the nomenclature adopted by the SOLTEN maintenance teams, henceforth the term \"street\" will be used to refer to both a row of PV units and the transiting path that lies at the south of each row. The inverters used in all the PV production units are Teide 100 [31], developed and manufactured by ITER. Teide 100 is a 100 kW rated, transformer-less inverter, especially designed for facilitating its operation and maintenance, as well as to contribute to maintain the grid through the voltage drop during exceptional conditions, such as voltage dips, according to the Spanish Grid Codes. The PV production units are connected to a SCADA (Supervisory Control and Data Acquisition) system, which monitors the inverters; the security equipment; the energy meters; and, since 2011, also a weather station located at the facilities that has, among other sensors, a ISO 9060 spectrally flat Class B pyranometer (CMP6 by Kipp and Zonen), which was placed, oriented, and tilted as the PV plants. All the acquired data are securely stored in a database. As can be seen in Figure 3, SOLTEN 20 MW is divided in two phases: SOLTEN I (13 MW) and SOLTEN II (7 MW), which differ only in the way in which the energy injected into the grid is accounted for. Thus, while SOLTEN I is articulated around 130 PV units, each one serviced by a class B in active energy meter (according to European Directive MID EN 50470 or class 1 according to IEC-62053-2) that is connected to the low-voltage grid, SOLTEN II has 70 PV units that are all serviced by one single energy meter instead, which is connected to the medium voltage grid. Because of this difference, the present study had to be restricted only to the SOLTEN I phase. The reason behind this configuration around 100 kW production units has to do with the kind of business model applied during the promotion and construction of the facilities, which was conceived in order to make use of the most suitable feed-in tariffs available at the time [32]. Thus, the installation was articulated as a collective solar farm,  
Figure 1: Geographical location of the SOLTEN PV installation (own elaboration).