Cities are critical actors in reducing global greenhouse gas (GHG) emissions; the level of knowledge, skills, and capacity, especially regarding innovative project planning such as sustainable energy (SE) projects, is fundamental for cities to meet their untapped potential. A range of indicator-based frameworks have been developed, focusing on general city economic competitiveness, yet these are often disconnected from the actual city operation and processes relevant to the investment cycle of SE projects. This paper proposes an integrated methodology for developing a City Capability Assessment Framework focusing on city planning, financing, and implementation capacity for SE projects. A principal component analysis (PCA) was applied to the results of a European city-survey to uncover areas highly relevant for the successful implementation of local SE projects. The insights gained aid policymakers to better understand implementation barriers, leading to the development of more effective actions and policy interventions towards the implementation of SE projects. The proposed framework can further enhance cities’ efforts with the implementation of local SE projects and guide potential investors to properly evaluate city capabilities and potential, when choosing local investment projects.