## 1 Introduction  
Predicting the electric energy that a photovoltaic (PV) system can produce over a certain period has always been key to PV designers and promoters for ensuring the supply stability and surveillance of PV plants. Meanwhile, forecasting the performance of PV modules in an urban context, particularly building-integrated photovoltaic (BIPV) systems, which entail additional difficulties compared to standard PV plants, has gained attention in the last few years in the framework of net-zero energy buildings to ensure functional system designs that work under suboptimal conditions [1; 2]. New developments in BIPV modules are increasingly demanding tools to determine the best option for each case to balance design and PV production [3; 4; 5; 6]. Boundary conditions that need to be considered in an accurate performance analysis of a BIPV system are, e.g., highly probable occurrence of partial shading, reduced or no backside ventilation of BIPV modules, higher angular losses due to the position of the generators, or soiling in non-accessible building areas. PV in buildings is key in the transition towards improved energy efficiency and decarbonisation, as it can aid in compensating for global energy consumption and emissions associated with buildings. Moreover, BIPV is the option that best combines on-site PV generation with building architecture by replacing conventional construction materials with BIPV elements, which helps achieve more sustainable and resilient buildings [7; 8]. All this aligns with the European Union's strategies on energy efficiency and the use of renewable energies to achieve net-zero greenhouse gas emissions in 2050, resulting in Europe becoming the first climate-neutral continent [9]. BIPV applications can be classified into several groups based on the location and position of the modules within the building envelope, as well as the accessibility from within the building [10; 11]. A summary of the most common BIPV applications is presented in Figure 1.  
The constructive system for BIPV installations directly affects PV performance. Factors such as rear-side ventilation or thermal insulation deeply impact the module's temperature [12]. The mounting structure and position of the module (vertical, horizontal or tilted) influence back airflow for the system's ventilation. A main concern in BIPV designs is predicting the system's actual output energy after accounting for all possible limiting factors and losses under operating conditions. The energy generated by a PV system, measured in kWh, is mainly a linear function of the solar irradiation received by the PV generator, also modified by secondary factors that affect PV power and cause losses in the system's output energy. The coefficient for the system's efficiency that accounts for second-order corrections due to non-ideal boundary conditions is defined as the performance ratio (_PR_). The performance ratio has been widely used to assess the performance of PV systems, and it is usually included in the monitoring schemes of operation and maintenance plans to check systems' performance evolution and health [13]. The _PR_ can be interpreted as the index that accounts for the impact of all losses in the system's output. These losses include temperature, angular, solar spectrum mismatch, or soiling effects, among others,  
Figure 1: Main BIPV applications. plus system component inefficiencies or failures, named the balance of system losses, mainly due to the inverter, wiring, and junction boxes, as indicated in the IEC 61724-1 Standard [14]. This IEC Standard describes how to obtain the \(PR\) for an existing PV system with experimentally measured data for a given period (day, month, year). Several studies have analysed the performance ratio of different PV systems and technologies for existing monitored installations worldwide, as summarised in [15], with annual \(PR\) values increasing from around 0.6 for older systems to over 0.9 in recent cases.