As cities worldwide pursue climate neutrality, Positive Energy Districts (PEDs)—urban areas that generate more energy than they consume—have become a focal point of sustainable urban/energy planning. Yet, assessing PED potential at the district scale remains a challenge due to data silos, fragmented data models and a lack of interoperable tools. This paper presents a modular, standards-based urban digital twin workflow that integrates detailed building-level energy simulations with district-scale energy balance analysis to evaluate PED potential. Centered on open-source/freeware tools and OGC standards, our approach leverages CityGML enriched with Energy ADE 2.0—a new and improved version of Energy ADE 1.0 currently in development—and couples SimStadt simulations with MAPED district assessments. All data and results are embedded in a 3D city database using PostgreSQL and visualised interactively through a web-based platform using OGC 3D Tiles and WFS. A real-world test in Stuttgart’s Nordbahnhofviertel district demonstrates the framework’s capability to generate actionable energy insights and confirm the area’s current shortfall in PED readiness. The methodology is now being replicated in Vienna, Rotterdam, and Wrocław, showcasing its scalability and utility for evidence-based energy planning across Europe.