This talk focuses on addressing the pressing need for renewable energy production in urban areas and cities, given their significant energy consumption and vulnerability to climate change. Luxembourg, in particular, has set ambitious goals for reducing greenhouse gas emissions and increasing renewable energy production. However, public authorities often lack the expertise and tools to make informed decisions about energy strategies. The presentation introduces the concept of an interoperable geographical digital twin based on free and open-source software and geospatial technologies. This digital twin integrates 3D CityGML data with simulation algorithms for renewable energy potentials and the energy grid. It aims to simulate the potential for solar photovoltaic (PV) electricity generation in cities, considering factors such as building characteristics and solar orientation. Results from this platform provide valuable insights for municipalities, urban planners, and citizens to support realistic urban energy planning. By offering high-resolution information on PV power generation at various scales, it aids in the development of sustainable energy plans and facilitates cost-efficient PV placement in buildings. In conclusion, this presentation highlights the importance of geographical digital twins in advancing the transition from fossil fuels to renewables. An interoperable digital twin, built on open-source technologies and open data, provides a flexible and collaborative environment for simulating and testing energy transition scenarios, ultimately accelerating the adoption of renewable energy in urban areas.