## 3 Results and Discussion  
Figure 1 shows the total irradiance values received on the plane of the PV modules, which were recorded every 5 min during the entire period analysed in this work, from January 2011 to February 2020. There are two periods, in 2012 and 2016, where, due to failures in the monitoring system, no data records were available, and which, as indicated above, were excluded from the classification process. In the colour map represented in Figure 1, it can be seen that the maximum daily irradiance values logically occur in the central hours of the day, between 12 p.m. and 4 p.m., and these values are higher in the summer months. It can also be seen how the irradiance values decrease from the maximum values in the central hours of the day, towards the hours of sunrise and sunset, with obviously zero irradiance in the night hours. The greater number of hours of sunshine in the summer months in the geographical location where the measurements were taken is evident. It can also be observed how, on days with passing clouds, there are variations or fluctuations in the irradiance values throughout part or all the day. The daily irradiance profiles with non-zero values shown in this figure are therefore those that were used for the classification carried out in this work. Figure 2 shows a histogram in which the total daily irradiance values for each of the days shown in Figure 1 can be observed. It can be seen that, influenced by the greater or lesser number of daily hours of sunshine throughout the year, and by the meteorological conditions, the total daily irradiance received by the panels in the geographical location analysed in this work can vary from values of less than 10 to values greater than 80 kW/m\({}^{2}\) per day. The red dashed lines show the values of total daily irradiance received by the panels that were obtained by the k-means algorithm to classify the analysed days in the three types named low (L), medium (M) and high (H). Figure 1: Instantaneous in-plane irradiance values registered from 2011 to 2020 with a frequency sample of 5 min. Figure 3 shows, for each of the days of the period analysed, the average daily efficiency values of the set of 36 PV modules studied, connected to the same inverter, and organised, as mentioned above, into three parallel strings, each with 12 modules connected in series. The efficiency values of the modules represented in this figure were obtained by averaging for each day the efficiency values (\(\eta_{C}\)) determined by Equation (3), which were calculated every 5 min. It can be seen that, for the days with the irradiance profiles shown in Figure 1, the set of panels worked on average with efficiencies ranging from around 8% to values of over 15%, so that this daily operating parameter of the modules can present variations of up to 8%. With regard to the operation of the inverter, the daily average efficiency values for the period of days analysed are shown in the histogram in Figure 4. The efficiency values of the inverter represented in this figure were obtained by averaging for each day the efficiency values (\(\eta_{inv}\)) determined by Equation (6), which were calculated every 5 min. It can be seen that, in this case, the inverter under study can operate with an efficiency ranging from values of around 75% to values close to 95%, although the values with the highest incidence were above 90%. The European weighted performance of the inverter, given by its manufacturers, is 95.1%, and 96% is the maximum efficiency, as reflected in Table 2. These values were not reached in Figure 4, since the daily average values of this parameter  
Figure 3: Histogram corresponding to the daily average efficiency values of PV modules during the period 2011–2020. Figure 2: Histogram corresponding to the daily total values of irradiance recorded on the plane of the PV modules during the period 2011–2020. are shown. However, efficiency values below 90% indicate that the inverter is working below its proper operating state.