## 3 Results and Discussion
And in the case of the so-called performance ratio (_PR_) (Equation (8)), the daily average values for the period analysed are shown in the histogram in Figure 5. It can be seen that it varies between values above 0.3 and values very close to 1, although for the data set represented, the daily average _PR_ values that determine the interquartile range (Q1-Q3) are 0.79 and 0.87, and 0.83 is the median of those data. It can be seen, then, that once the adopted configuration of the modules and the inverter associated with them is fixed by the design and dimensioning of the set, their operation presents margins of variation, which shows how these components respond to changes in the meteorological conditions. In this way, the energy production will not be exclusively proportional to the irradiance received, but the variation in the meteorological conditions affects the operation performance of the PV installation components, which can lead to a variation in the expected production. With respect to the first classification carried out in this work, obtained by applying the _k-means_ algorithm to all the total daily irradiance values of the 3237 days studied, the results obtained are shown in Figure 6, where the number of days obtained in each of  
Figure 4: Histogram of daily average values of inverter efficiency over the period 2011–2020. Figure 5: Histogram corresponding to the daily average values of the performance ratio (_PR_) for the inverter together with the associated modules over the period 2011–2020. the three types is represented, as well as the values of the total daily in-plane irradiance received in the modules for each of these days. Of the total number of days analysed, 550 days were obtained belonging to the low type (which represents 17.0% of the total number of days analysed), 1043 corresponded to the medium category (which represents 32.2%), and 1644 corresponded to the high category (which represents 50.8%). The total daily irradiance received on the modules on L days was between 2.5 and 35.8 kW/m\({}^{2}\), with an average value of 21.3 kW/m\({}^{2}\). For M days it was between 35.8 and 61.2 kW/m\({}^{2}\), with an average value of 50.4 kW/m\({}^{2}\), and for H days, the range of the total daily irradiance values varied between 61.2 and 81.9 kW/m\({}^{2}\), with an average value of 72.1 kW/m\({}^{2}\). This implies that on L days, the total in-plane irradiation received by the PV panels was between 0.2 y 3.0 kWh/m\({}^{2}\) (an average value of 1.8 kWh/m\({}^{2}\)); between 3.0 and 5.1 kWh/m\({}^{2}\) (an average value of 4.2 kWh/m\({}^{2}\)) on M days; and between 5.1 and 6.8 kWh/m\({}^{2}\) (an average value of 6.0 kWh/m\({}^{2}\)) on H days. Of the total irradiance or irradiation received by the panels during the nine years of monitoring considered in this work, 6.4% would have been received on L days, 29.2% would have been received on M days, and 64.4% would have been received on H days. Once this first classification was carried out, the second classification was applied, from which a total of nine types of days or clusters were obtained, with different characteristics from one to another, in terms of the amount of irradiance and its variation or fluctuation throughout the day. A sample of the daily irradiance profiles obtained in each of the nine different types of days or clusters is shown in Figure 7. The centroids corresponding to each of the nine clusters obtained are shown in Figure 8. In addition, the values of total in-plane daily irradiance, for each of the days obtained in each of the nine subcategories, are shown in Figure 9, where the number of days obtained in each one is also shown on x-axes. The mean values of total daily irradiance for days H1, H2 and H3 were, respectively, 72.4, 72.0 and 67.8 kW/m\({}^{2}\). The corresponding values for days M1, M2 and M3 were 51.5, 48.5 and 47.8 kW/m\({}^{2}\), and those of days L1, L2 and L3 were of 12.9, 25.6 and 27.9 kW/m\({}^{2}\), respectively.