## 4 Results and Discussion
### Performance Comparison between the Monthly and Annual Averages
\begin{table}
\begin{tabular}{l l c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Month Factors**} & \multicolumn{8}{c}{**2018**} & \multicolumn{8}{c}{**2019**} \\ \cline{3-14}  & & **4** & **5** & **6** & **7** & **8** & **9** & **10** & **11** & **12** & **1** & **2** & **3** \\ \hline
1 & SP & & & & & & & & & & & & & \\ \hline
2 & SP, SR & \(\surd\) & \(\surd\) & & \(\surd\) & & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) \\ \hline
3 & SP, Sunlight & \(\surd\) & \(\surd\) & \(\surd\) & & \(\surd\) & & & & & \(\surd\) & & \\ \hline
4 & SP, Humidity & & & & & & & & & & & & \\ \hline
5 & SP, Temperature & & & \(\surd\) & & \(\surd\) & & & & & & & \\ \hline
6 & SP, CC & & & & & & & & & & & & \\ \hline
7 & SP, WS & & \(\surd\) & \(\surd\) & & \(\surd\) & & & & & & & \\ \hline
8 & SP, SR, Sunlight & & \(\surd\) & \(\surd\) & & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) \\ \hline
9 & SP, SR, Humidity & & & \(\surd\) & & & & & & & & \(\surd\) & \\ \hline
10 & SP, SR, Temperature & & & & & & & & & & & \\ \hline
11 & SP, SR, CC & & & & & & & & & & & \\ \hline
12 & SP, SR, WS & \(\surd\) & \(\surd\) & \(\surd\) & & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) \\ \hline
13 & SP, SR, Sunlight, Humidity & & & & & & & & & & & \\ \hline
14 & SP, SR, Sunlight, Temperature & & & & \(\surd\) & & & & & & & \\ \hline
15 & SP, SR, Sunlight, CC & & & & & & & & & & & \\ \hline
16 & SP,SR, Sunlight, WS & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) \\ \hline
17 & SP, SR, Sunlight, Humidity, Temperature & & & & \(\surd\) & & & & & & & \\ \hline
18 & SP, SR, Sunlight, Humidity, CC & & & & & & & & & & & \\ \hline
19 & SP, SR, Sunlight, Humidity, WS & \(\surd\) & \(\surd\) & & & & & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \\ \hline
20 & SP, SR, Sunlight, Humidity, Temperature, CC & & & & & & & & & & \\ \hline
21 & SP, SR, Sunlight, Humidity, Temperature, & & & \(\surd\) & \(\surd\) & & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) \\ \hline
22 & SP, SR, Sunlight, Humidity, Temperature, & & & \(\surd\) & \(\surd\) & & \(\surd\) & & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Monthly performance comparison among the multivariate models. Figure 8: The meteorological factors affecting solar power forecasting by month. Tables 6 and 7 are the performance comparisons of the traditional LSTM and the proposed LSTM, respectively, for medium-term and long-term solar power forecasting by the models. Table 6 is the result of comparing the performance of medium-term solar power forecasting for April, which represents the spring season in Korea. \(\Delta\) is the difference between traditional LSTM and proposed LSTM. The medium-term solar power forecasting results for August, October, and January, included in Tables A2-A4 of Appendix B, represent the Korean summer, autumn, and winter, respectively. Figures 9 and 10 show the results of 22 models for the proposed and traditional LSTM by month (January, April, August, and October). The 22 models of the proposed LSTM forecast values similar to the observed values, but the 22 models of the traditional LSTM predict values that differ from the observed values. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No**} & \multicolumn{3}{c}{**RMSE**} & \multicolumn{3}{c}{**MAE**} & \multicolumn{2}{c}{**Computation**} \\ \cline{2-7}  & **Traditional** & **Proposed** & \(\Delta\) & **Traditional** & **Proposed** & \(\Delta\) & **Time (s)** \\ \hline
1 & 35.7 & 9.23 & \(-\)26.47 & 26.03 & 5.83 & \(-\)20.20 & 69.06 \\
2 & 4.83 &  
Figure 11: _Cont_. Figure 10: Comparison among the 22 models for the proposed LSTM by month.