This work introduces the first validated, user-friendly, and accurate open-source photovoltaic-thermal (PVT) collector model in Modelica, tailored for system-level simulation and optimization. Current state-of-the-art PVT collector Modelica models are largely limited to oversimplified, steady-state representations that fail to capture the dynamic thermal behavior inherent to real PVT systems. A comprehensive Modelica model is developed based on the ISO 9806 standard (test method for the quasi-dynamic thermal performance of solar thermal collectors), coupled with an electrical system model through an internal heat transfer coefficient. The model calibration relies exclusively on manufacturer datasheet parameters, thereby eliminating the need for parameter estimation from measurement data. The model is validated using experimental results from an unglazed PVT collector, demonstrating strong agreement for various (weather) conditions. The findings highlight that, while steady-state models may suffice for conventional solar thermal collectors (STCs), accurate PVT modeling necessitates a dynamic approach, particularly for the thermal aspects. The electrical output of the PVT collector is less sensitive to transient effects. In addition to the model formulation and validation, this work presents a user-friendly automated calibration method based on manufacturer data, and critically addresses the limitations and potential trade-offs of using exclusively datasheet-derived parameters, thereby providing a transparent tool for PVT system simulation, design, and optimization within the open-source IDEAS library.