Urban planning for renewable energies requires dynamic simulations to visualize complex spatial data to support stakeholders’ decision-making process. Tangible User Interfaces (TUIs) support interactions with complex data and provide a physical exploration ground. We designed a prototype for urban planning, integrating tangible windmills reacting in real-time to wind simulation. We conducted a within-subjects study (n=16), testing a tangible prototype against a graphical version. We measured perceived workload and usability, conducted semi-structured interviews, and collected observations during two tasks. Our results show that participants preferred the TUI over the Graphical User Interface (GUI), while reporting significant equivalent mental workloads. The participants felt as effective in the planning tasks with the TUI as with a GUI version. The TUI version enables more intuitive and direct interaction with digital city models, and working haptically was perceived as pleasant. Our findings contribute to the future work of TUI planning tools concerning renewable energy.