## 2 Experimental Setup and Proposed Method  
In this study, sizing and techno-economic analysis of utility-size PV systems with battery energy storage in commercial buildings according to Gerede/Bolu meteorological conditions were used. Gerede is located at a longitude of 32.70 E, a latitude of 40.8 N, and an altitude of 1350 m. A total of 1418 panels with a power of 455 Wp and 5 inverters with a power of 110 kW were used in the PV system. The PV system was connected to the electricity grid on 1 January 2023. The PV panels were placed on both the south and north facades following the building roof slope. The tilt angle of the panels was approximately 8 degrees, and the azimuth angle was roughly 25 degrees. In Figure 1, the photograph of the PV system installed on the roof of the commercial building is provided. Each inverter used in the power plant was connected to 16 PV arrays. Each PV array had 18 PV panels connected in series. However, some arrays had 17 PV panels. Solar cables of 290 m from the farthest string to the inverter and 130 m from the nearest string to the inverter were used. The design was developed so that the voltage drop in the cables would be a maximum of 1.5%. The voltage drop on the line with the longest distance was calculated as 1.45%. The single-line diagram of the PV plant with an Energy Storage System is shown in Figure 2.  
The power of the PV panel is especially affected by irradiation and temperature. The output power of a panel is directly proportional to radiation and inversely proportional to temperature. Therefore, the panel inclination angle, which affects the amount of radiation falling on the panel surface, and the distance of the panel to the roof, which affects the panel cell temperature, are two important factors. Seraphim SRP-455-BMA-HV panels were used in this study. The specifications of the panels are shown in Table 1.  
The total energy demand of the factory where this study was carried out in 2023 was 1,163,456 kWh. The PV power plant started producing energy on 1 January 2023. Electrical quantities such as produced energy, current, voltage, and power, as well as meteorological data such as radiation and temperature, were continuously recorded. Monthly demand and produced energy are shown in Figure 3.  
Figure 1: Experimental setup and installed 645 kWp PV system in Gerede/Bolu, Türkiye. Figure 2: Single-line diagram of the power plant. The PV power plant produced a total of 755,518 kWh of energy between 1 January 2023 and 1 January 2024. Accordingly, approximately 64.9% of the annual energy demand was produced by the PV plant. In particular, in the winter months, the rate of meeting the demand for energy from the produced energy was very low. However, the energy produced during the summer months was higher than the energy demanded. Therefore, there is a need for an energy storage system in both winter and summer months. The meteorological data were collected from the real-time remote reading system. The amount of energy that the PV system will produce largely depends on solar irradiation and PV cell temperature. Therefore, irradiation, cell temperature, and ambient temperature data are measured in the PV system. In this study, irradiation measurements were performed using a monocrystalline silicon (mono-Si) reference cell made of the same material as the PV modules used in the power plant. In order to ensure the accuracy of the measurements, the reference cell was calibrated in accordance with IEC 60904-2 standards [51]. The reference cell was placed at the same inclination angle and orientation as the PV panels. This arrangement minimizes measurement deviations that may arise from angular differences, ensuring reliable irradiation data are collected. In the PV system, meteorological and electrical data were measured at 5-min intervals. The reference cell was used for irradiation measurement, and the PT1000 (resistance) temperature sensor was used for panel cell temperature measurement.