## 3 Results and Discussion  
Daily values of global solar radiation, averaged for 11 years for each of the 15 stations, were the basis for the equations coefficients calibration (Table 2). The goodness-of-fit of the models (1)-(3) expressed by means of the determination coefficient R\({}^{2}\) did not vary considerably between the stations and ranged from 0.94 (Lesko) to 0.97 (Kolobrzeg). Although these values are slightly less favorable than in the other climate zones [22; 23; 24; 25; 26; 27; 28], the results indicate that the model is well-fitted to the averaged global solar radiation data. Furthermore, the model's precision was assessed by means of statistical indicators such as RMSE, MABE, and MAPE. Their values, collected in Table 3, were calculated using a separate, independent dataset. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Station** & **Model** & **a** & **b** & **c** & **d** & **e** & **f** & **g** & **R\({}^{2}\)** \\ \hline \multirow{4}{*}{Jarczew} & 1 & 1.44 & 19.25 & 10.45 & 2.11 & & & & 0.96 \\  & 2 & 10.86 & \(-\)9.68 & 10.45 & & & & 0.96 \\  & 3 & 11.13 & \(-\)9.81 & 1.03 & \(-\)4.21 & \(-\)3.95 & 1.03 & 4.94 & 0.96 \\  & 1 & 1.09 & 20.10 & 11.75 & 2.34 & & & 0.97 \\ Kolobrzeg & 2 & 10.54 & \(-\)10.19 & 11.55 & & & & 0.96 \\  & 3 & 11.05 & \(-\)8.41 & 1.06 & \(-\)3.98 & \(-\)6.59 & 1.05 & 5.36 & 0.97 \\  & 1 & 1.97 & 19.36 & 10.83 & 2.17 & & & 0.96 \\ Legnica & 2 & 11.34 & \(-\)9.76 & 10.77 & & & & 0.96 \\  & 3 & 11.65 & \(-\)6.82 & 1.04 & \(-\)3.76 & \(-\)7.43 & 1.03 & 5.60 & 0.96 \\  & 1 & 2.3 & 17.91 & 8.51 & 2.14 & & & 0.94 \\ Lesko & 2 & 11.03 & \(-\)9.02 & 8.50 & & & & 0.94 \\  & 3 & 11.29 & \(-\)6.29 & 1.03 & \(-\)3.69 & \(-\)7.42 & 1.03 & 5.57 & 0.94 \\  & 1 & 1.03 & 19.38 & 13.51 & 2.35 & & & 0.96 \\ Leba & 2 & 10.12 & \(-\)9.83 & 13.19 & & & & 0.96 \\  & 3 & 10.54 & \(-\)7.10 & 1.06 & \(-\)3.79 & \(-\)7.51 & 1.04 & 5.59 & 0.96 \\  & 1 & 1.46 & 18.59 & 10.74 & 2.17 & & & 0.96 \\ Lódáz & 2 & 10.46 & \(-\)9.37 & 10.68 & & & & 0.96 \\  & 3 & 10.74 & \(-\)5.22 & 1.04 & \(-\)3.54 & \(-\)8.33 & 1.03 & 5.79 & 0.96 \\  & 1 & 1.11 & 19.89 & 11.75 & 2.15 & & & 0.96 \\ Mikolajki & 2 & 10.78 & \(-\)10.02 & 11.72 & & & & 0.96 \\  & 3 & 11.10 & \(-\)4.72 & 1.04 & \(-\)3.50 & \(-\)8.92 & 1.03 & 5.89 & 0.96 \\  & 1 & 1.11 & 18.97 & 11.66 & 2.17 & & & 0.96 \\ Pila & 2 & 10.30 & \(-\)9.56 & 11.57 & & & & 0.96 \\  & 3 & 10.56 & \(-\)3.21 & 1.06 & \(-\)3.31 & \(-\)9.40 & 1.03 & 6.06 & 0.96 \\  & 1 & 1.68 & 19.25 & 5.97 & 2.36 & & & 0.96 \\ Pulawy & 2 & 10.69 & \(-\)9.76 & 6.03 & & & & 0.96 \\  & 3 & 11.37 & \(-\)0.93 & 0.98 & \(-\)3.17 & \(-\)9.36 & 1.08 & 6.04 & 0.96 \\  & 1 & 1.07 & 18.03 & 11.7 & 2.12 & & & 0.96 \\ Torun & 2 & 9.88 & \(-\)9.07 & 11.63 & & & & 0.96 \\  & 3 & 10.06 & \(-\)1.89 & 1.08 & \(-\)2.93 & \(-\)9.64 & 1.02 & 6.22 & 0.96 \\  & 1 & 1.27 & 19.3 & 10.05 & 2.2 & & & 0.96 \\ Warszawa & 2 & 10.57 & \(-\)9.74 & 10.00 & & & & 0.96 \\  & 3 & 10.95 & \(-\)0.85 & 1.07 & \(-\)2.84 & \(-\)9.91 & 1.04 & 6.25 & 0.96 \\  & 1 & 1.86 & 19.36 & 10.7 & 2.14 & & & 0.96 \\ Wielur & 2 & 11.28 & \(-\)9.75 & 10.64 & & & & 0.96 \\  & 3 & 11.79 & \(-\)0.57 & 2.54 & \(-\)6.99 & \(-\)9.44 & 1.05 & 6.32 & 0.97 \\  & 1 & 1.48 & 19.41 & 8.75 & 2.14 & & & 0.96 \\ Wlodawa & 2 & 10.93 & \(-\)9.78 & 8.77 & & & & 0.96 \\  & 3 & 11.07 & \(-\)0.49 & 3.09 & \(-\)8.48 & \(-\)9.74 & 1.02 & 6.39 & 0.96 \\ Poland & 1 & 1.41 & 19.19 & 10.50 & 2.19 & & & 0.96 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._RMSE varies slightly by the model, but considerably by location. For 8 out of 15 stations, the difference of RMSE between models did not exceed 0.02 MJ-m\({}^{-2}\), with the exception of Pulauwy, where it counts for 0.12 MJ-m\({}^{-2}\). The best accuracy in terms of RMSE was obtained for Model (3) for Legnica station: 0.45 MJ-m\({}^{-2}\) (0.46 MJ-m\({}^{-2}\) and 0.47 MJ-m\({}^{-2}\) for Models (1) and (2), respectively). The highest RMSE, 1.10 MJ-m\({}^{-2}\), was noted for Model (2) at the Leba station (1.02 MJ-m\({}^{-2}\) and 1.01 MJ-m\({}^{-2}\) for Model (1) and (3), respectively).