## 2 Materials and Methods
### Experimental Setup
Meteorological station (**a**), PV modules under test (**b**), and back of bPV with high-reflecting ground surface (**c**). The post-process of the curves consisted of 3 steps:  
1. Each set of five curves was translated to the mean irradiance of the event, assuming a constant temperature, as in Procedure 4 of Standard IEC 60891: 2021 [35], shown in Equations (1)-(4): \[I_{1}^{\prime} =I_{1}+I_{sc,1}\left(\frac{G_{2}}{G_{1}}-1\right),\] (1) \[V_{1}^{\prime} =V_{1}-R_{s}(I_{1}^{\prime}-I_{1}),\] (2) \[I_{2} =I_{1}^{\prime}+\alpha(T_{2}-T_{1}),\] (3) \[V_{2} =V_{1}^{\prime}+\frac{T_{2}-T_{1}}{T_{1}}(V_{1}^{\prime}-N_{c} \cdot\varepsilon)\] (4) where subscripts 1 and 2 denote the original and target conditions, respectively. The number of cells connected in series \(N\), and the current temperature coefficient \(\alpha\) can be seen in Table 1. The internal series resistance \(R_{s}\) and the constant \(\varepsilon\), which is independent of temperature and irradiance, were fitted to obtain maximum power and voltage errors within \(\pm\)0.5% at STC in all the curves. Their values are 0.45 \(\Omega\) and 1.2936 V, respectively. 2. The curves were resampled at identical voltage values and then averaged to obtain a single smoother curve. 3. Finally, for the averaged curve, the main characteristic values were obtained: \(I_{sc}\), \(I_{mp}\), \(V_{oc}\), \(V_{mp}\), and \(P_{mp}\). The values of \(I_{sc}\) and \(V_{oc}\) were obtained by local fittings in the region close to short- and open-circuits. In the case of \(I_{sc}\), linear regression was used for voltage under 10 V. For \(V_{oc}\) by second order polynomial fitting in the region of \(V>V_{mp}\). An example of the 5 original \(I\)-\(V\) curves and the final one after post-process can be seen in Figure 2.  
To compare the performance of the three modules (PV1, PV2, and bPV) with different sizes and characteristics, the values were normalized by taking into account the technical features provided by the manufacturer (see Table 1). Therefore, this normalization ensures an equitable and scientifically rigorous analysis, which allows a fair comparison between PV modules with different characteristics and rated power, through the adoption of normalized indexes, as recommended by international standards [36]. Moreover, PV technologies with different characteristics were selected to reflect the diversity of options available in the \begin{table}
\begin{tabular}{l c c} \hline
**Equipment** & **Manufacturer-Model** & **Other** \\ \hline _1-V_ Tracer & TeamWare-Wally & Voltage, current precision: \(\pm\)0.5\% (2\text{--}120\%\) full scale range) \\ \hline Thermocouple modules & RTD PT100 A Class & Accuracy: 0.25 \({}^{\circ}\)C; Resolution: 0.1 \({}^{\circ}\)C \\ \hline Data Logger & LSI-Elo3305 & Acquisition rate: 1 Hz \\ \hline Thermocouple humidity sensor & LSI-DMA875 & \begin{tabular}{c} Pluto 10/3 B (DIN EN 60751); \\ Measurements range: [-30 \({}^{\circ}\)C, \\ +70 \({}^{\circ}\)C]; Resolution: 0.04 \({}^{\circ}\)C; Uncertainty: 0.2 \({}^{\circ}\)C (at 0 \({}^{\circ}\)C) \\ \end{tabular} \\ \hline Pyranometer (GHI and GTI) & LSI-DPA252 & \
\begin{tabular}{c} ISO9060 Secondary standard; \\ Spectral range: 285-3000 nm; \\ Response time: 4.5 s; Directional \\ response \(\leq\)\(\pm\)10 W/m\({}^{2}\) \\ \end{tabular} \\ \hline Pyranometer (DHI) & LSI-DPA154 &
\begin{tabular}{c} ISO9060 First Class; Spectral \\ range: 285-3000 nm; Response \\ time: 16 s; Directional response: \\ \(<\)\(\pm\)20 W/m\({}^{2}\) \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 2: Technical characteristics of measurement devices at SolarTech\({}^{\text{LAB}}\). market and to provide a more comprehensive analysis of their performance under various conditions. This research aimed to highlight the practical implications and real-world performance differences that users might encounter, with the reported experimental data. As a final step, the bifacial contribution of bPV was found by subtracting the estimated monofacial \(P_{mp}\) from the experimental value.