## 3 Results and Discussion
### Performance Parameters
Results of the ANOVA, which are related to an analysis of variance of POA radiation on PV array by \"months\" groups, are shown in Table 4, and a \(p\) value less than 0.001 shows that at least the mean between two groups of data is statistically significantly different. However, with ANOVA it is not clear which of them are significantly different. For that reason, the post hoc Tukey test is performed (Table 5). As was expected, the mean of POA radiation has the highest values on July and August. Within the Table 5, the months which are not statistically significantly different are marked with the same letter, while those that are statistically significantly different have letters that are also different. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source** & **DF** & **Sum of Squares** & **Mean Square** & **F Ratio** & **Prob \textgreater{} F** \\ \hline Month & 11 & 81,047,311 & 7,367,937 & 85.1508 & \textless{}0.0001 \\ Error & 100 & 8,652,807 & 86,528 & & \\ C. Total & 111 & 89,700,118 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Analysis of variance of POA radiation on PV array by “months” groups. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Level** & & & **Mean** \\ \hline July & A & & & 3204.6841 \\ August & A & & & 3194.4330 \\ June & B & & & 2715.8129 \\ May & B & & & 2629.4805 \\ September & B & & & 2470.4103 \\ April & B & & & 2409.3351 \\ Marth & & C & & 1918.8154 \\ October & & C & & 1810.7883 \\ February & & D & & 1158.8864 \\ November & & D & E & 1054.8455 \\ January & & & D & E & 776.5689 \\ December & & & & E & 667.1575 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The results of the Tukey test—connecting letters report. Figure 9: POA radiation on PV array during the months measured over the 10-year period. It should be noted that levels not connected by the same letter are significantly different. The values of PV electricity during the 10-year period are also analyzed and presented in Figure 10.  
In Figure 10, the black line is the mean of the data. The top and bottom of the green diamonds represent the confidence interval (1-alpha) \(\times\)100 for each year (group). The middle line across the diamond represents the mean of the group (mean of the PV electricity within the specific year). The height of the diamonds is proportional to the reciprocal of the square root of the number of observations in the groups. The horizontal extent of each group along the horizontal axis is proportional to the sample size for each level of year variable. It means the narrower diamonds are usually taller. The mean line across the middle of each diamond represents the group mean (mean of PV electricity of the year). Considering that the variable \"years\" is a categorical variable and \"PV electricity\" is numerical one, a one-way analysis needs to be performed. In this regard, it is necessary to test the conditions for applying ANOVA. The O'Brien test is used to check the equality of variances. The obtained \(p\)-value of 0.9860 in the O'Brien test indicates that it cannot be said that the variance along the groups (the group is the PV electricity values for a specific year and it means that the sample consists of 10 groups/years) is not equal and ANOVA can be used. The analysis of variance of the PV electricity by years (groups) is given in Table 6.  
The ANOVA shows, with a \(p\)-value of 0.9999, that it cannot be stated that the mean values of PV electricity between the groups differ from each other. In other words, based on the data obtained over 10 years, it cannot be stated that the mean values of PV electricity change statistically significantly with the years. Because POA radiation and PV electricity are statistically speaking matched pairs data, it means that the same conclusion related to PV electricity could be applied on POA radiation in terms of mean difference over the 10 years.