An important prerequisite for the simulation-based assessment of energy systems at urban scale is the availability of high-quality, well-formatted and semantically structured data. Unfortunately, best practices and state-of-the-art approaches for urban data modelling are hardly applied in the context of energy-related simulations, such that data management and data access often become tedious and cumbersome tasks. This paper presents the so-called Simulation Package, i.e., a data model extending the 3D City Database for CityGML, and its derived data access layer, both aiming to bridge this gap between semantic 3D city modelling and simulation in the context of urban energy systems. The feasibility of this approach is demonstrated with the help of a concrete example, where the proposed extension has been implemented and integrated into a simulation toolchain. The aim is that the availability of a common, shared data model and the proof-of-concept implementation will contribute and foster adoption and further improvement in the future.