## 2 Materials and Methods
### Determination of the Performance Ratio  
The performance ratio is commonly calculated from the ratio of the final system yield (\(Y_{\mathrm{f}}\)) over the reference yield (\(Y_{\mathrm{r}}\)). The final system yield is the net output energy of the PV system per unit of installed PV array power; the reference yield is the ratio of the total irradiation to the reference irradiance (Equation (1)):  
\[PR=\frac{Y_{\mathrm{f}}}{Y_{\mathrm{r}}}=\frac{E_{\mathrm{out}}/P_{0}}{H_{ \mathrm{POA}}/G_{\mathrm{STC}}}=\frac{E_{\mathrm{out}}\cdot G_{\mathrm{STC}}}{ P_{0}\cdot H_{\mathrm{POA}}} \tag{1}\]  
where \(G_{\mathrm{STC}}\) stands for irradiance under standard test conditions (STCs), which are 1 kW/m\({}^{2}\) for plane-of-array irradiance (\(G_{\mathrm{POA}}\)), 25 \({}^{\circ}\)C for PV cell junction temperature, and 1.5 G for air mass solar spectrum, as defined in the IEC 60904-7 Standard [45]. \(E_{\mathrm{out}}\) is the output energy of the PV system; \(P_{0}\) is the PV array nominal power (maximum power under STCs), and \(H_{\mathrm{POA}}\) is the plane-of-array (POA) irradiation.  
Measured or estimated PV power and irradiance values over a specific period allow for the determination of PV energy and solar irradiation by time integration, commonly approximated by summation. The output energy can be estimated from \(G_{\mathrm{POA}}\) data (either experimental or simulated) and performance ratio estimations (Equation (2)):  
\[E_{\mathrm{out}}=\sum_{\mathrm{i}}\frac{P_{0}}{G_{\mathrm{STC}}}\cdot G_{ \mathrm{POA}_{\mathrm{i}}}\cdot\Delta t\cdot PR_{\mathrm{i}} \tag{2}\]  
where index i represents the iteration number for each time interval \(\Delta t\). The main challenge is accurately determining the value of the performance ratio at any time. Analytically, \(PR\) can be expressed as the product of different power loss factors, each one complementary  
Figure 2: BIPV modules are integrated into the upper pane of the ventilated facades of Building 42.  
to the corresponding power loss \(L_{\text{k}}\) (Equation (3)) [14]. The expression applies to any time interval:  
\[PR=\big{(}1-L_{\text{temp}}\big{)}\cdot(1-L_{\text{AOI}})\cdot\Big{(}1-L_{\text {solling}}\Big{)}\cdots(1-L_{\text{k}})=\prod(1-L_{\text{k}}) \tag{3}\]  
The power losses and performance ratio can be expressed as a fraction or percentage, with \(PR\) equal to 1 (or 100%) being the ideal situation with all \(L_{\text{k}}=0\). This paper will handle fractional values unless otherwise specified. Power losses related to factors such as, e.g., soiling, snow, and the balance of the system can be found tabulated in the literature [46]. However, losses such as temperature, angular, soiling, and spectral losses rely on environmental and design factors and require further analysis.  
In the present work, on the one hand, experimental \(PR\) calculations were performed for the year 2019 on an hourly basis using Equation (1). On the other hand, estimated \(PR\)s were obtained using CAMS data for irradiance values and PVGIS for ambient temperature and wind speed data using Equation (3). Calculating system losses for the estimated \(PR\) can pose different challenges depending on the type of PV system and installation, as well as the availability and quality of input data. This paper has attempted to address these challenges and provide a means to apply the methodology to other PV systems with different boundary conditions.