The citiwatts platform, an open-source toolbox developed during the European projects Hotmaps and OpenGIS for Energy Transition (OpenGIS4ET), supports energy planning amid the energy transition by integrating diverse energy systems and enabling flexible, data-driven analyses. citiwatts combines open-source and user-uploaded private data with privacy and security measures, enhancing planning precision. Its calculation modules (CMs) model complex energy scenarios like coupling electric vehicle (EV) recharge with photovoltaics (PV), allowing users to assess strategies such as EV charging coordinated with local PV generation. A case study in Sion, Switzerland, demonstrates how the platform can be used to compare the existing EV charging infrastructure with the recharging requirements to the horizon 2050. The secure, GDPR-compliant platform enables stakeholders, like local Distribution System Operators (DSOs), to integrate proprietary data and analyse energy demands at various scales. citiwatts emerges as a valuable tool for sustainable energy planning, offering policymakers and planners an adaptable solution aligned with CO2 reduction goals.