This paper addresses the significant challenge of modeling building integrated photovoltaic (BIPV) systems under partial shading (PS). Most of the existing photovoltaic (PV) system models struggle capturing accurate system performance when environmental conditions deviate from rated conditions, such as in the case of shading. Recognizing the complexity of accurately modeling BIPV system performance under low irradiance, we introduce and validate a modular electrothermal modeling approach, capable of easily capturing partial shading behaviors. The proposed PV system model is built as modules consisting of an array of PV cells connected to bypass diodes (BPDs). The electrical model of the PV cells is based on an eleven-parameter functional form single diode model (SDM) and the thermal model portion is based around measurable environmental conditions. BPDs are modeled through the Shockley equation along with a heat transfer model based on thermal resistances. The electrothermal model is validated using real PV array data at varying environmental conditions. The proposed model is also compared against an electrothermal model based on a five parameter functional form SDM, demonstrating superior performance in simulating the complicated behavior of BIPVs under low irradiance conditions. The enhanced model enables highly detailed, time-resolved shading analysis, offering a framework for future research into BIPV modeling and control.