# Photovoltaic Prediction Software: Evaluation with Real Data from Northern Spain
## 4 Results and Discussion
### Monthly Results  
In addition, a comparison with monthly results was also performed. In this way, the results could be analyzed in more detail, so as to establish whether the deviation was balanced throughout the year. The deviation percentage from the actual monthly values are shown in Table 5 for the three systems. So that the interpretation of the large volume of data is easier, the cells in this table are color-coded. Red cells show results that were overestimated, blue cells show the underestimated ones.  
An analysis of the monthly results showed that most models overestimated the energy generated during the winter months (when daylight hours and solar irradiation are lower), while they underestimated the results during summer, compensating the deviation throughout the year.  
The model for fixed installations (System 1), as previously mentioned, showed very good results. The models that showed better results were SAM isotropic and PVSYST new. However, the results were consistent for most months. From November to January the results showed high deviations.  
Despite the greater complexity of System 2 and therefore its higher operational uncertainty, due to possible orientation mismatch, in general, all the programs yielded accurate predictions in the warmer months. On the other hand, errors, even higher than 60%, were obtained during the winter months. In this case, the model with the best results was SAM. It can therefore be deduced that none of the climate databases were accurate in wintertime.  
Lastly, the results for System 3 were shown to be inconsistent. In this type of installation, the models provided very different values throughout the year. In May, the deviation increased in all the models. Spring is usually windy; therefore, the solar tracker automatically activated save mode and was not correctly oriented. RETScreen and PVGIS COSMO provided the best results during the different months of the year.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{8}{c}{**AC Energy Generated in a Year (kWh/kW)**} \\ \hline \multicolumn{8}{c}{**P1**} & \multicolumn{2}{c}{**T1**} & \multicolumn{2}{c}{**L1**} \\ \hline
**Actual** &
\begin{tabular}{c} **Average** \\ **Year** \\ \end{tabular} & \multicolumn{2}{c}{**1426 kWh/kW\({}_{\text{P}}\)**} & \multicolumn{2}{c}{**1435 kWh/kW\({}_{\text{P}}\)**} & \multicolumn{2}{c}{**1732 kWh/kW\({}_{\text{P}}\)**} \\ \hline
**Software** & **Options** & **kWh/ kW\({}_{\text{P}}\)** & **Deviation** & **kWh/ kW\({}_{\text{P}}\)** & **Deviation** & **kWh/ kW\({}_{\text{P}}\)** & **Deviation** \\ \hline
**RETScreen** & - & 1479 & 3.72\% & 1547 & 7.79\% & 1761 & 1.68\% \\ \hline \multirow{4}{*}{**SAM**} & _ISOTROPIC_ & 1359 & \(-\)4.73\% & 1435 & \(-\)0.04\% & 1627 & \(-\)6.05\% \\  & _HDKR_ & 1384 & \(-\)2.96\% & 1485 & 3.47\% & 1753 & 1.21\% \\  & _PEREZ_ & 1401 & \(-\)1.76\% & 1507 & 5.01\% & 1796 & 3.73\% \\ \hline \multirow{4}{*}{**PVGIS**} & _SARAH_ & 1439 & 0.93\% & 1617 & 12.68\% & 2073 & 19.73\% \\  & _CMSAF_ & 1387 & \(-\)2.73\% & 1581 & 10.15\% & 1983 & 14.55\% \\  & _ERAS_ & 1419 & \(-\)0.48\% & 1586 & 10.49\% & 2111 & 21.92\% \\  & _COSMO_ & 1302 & \(-\)8.70\% & 1427 & \(-\)0.56\% & 1752 & 1.19\% \\ \hline
**PVSYST** & _New_ & 1413 & \(-\)0.95\% & 1464 & 2.00\% & 1953 & 12.80\% \\  & _Classic_ & 1281 & \(-\)10.20\% & 1321 & \(-\)7.93\% & 1705 & \(-\)1.53\% \\
**PV*SOL** & - & 1580 & 10.80\% & 1590 & 10.82\% & 1855 & 7.12\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Yearly energy results comparative of each system.