Two-thirds of the overall primary energy consumption in the world occurs in urban settlements, and therefore cities represent a rich ground for CO2 emissions reduction. When developing energy strategies, for CO2 emissions reduction purposes, energy planners confront the complexity of cities. This is because these latter comprise a large variety of components and interdependencies. Furthermore, a large number of different stakeholders have conflicting interests in cities. This paper presents an ontology-based urban energy planning support methodology, applied in building-Integrated Solar PV. Based on a process meta-model of urban energy planning, the main characteristics if urban planning supports systems are derived. A modular methodology to develop such systems is defined. Then, an ontology is incrementally developed, fulfilling these characteristics, as an application of the defined methodology to building-integrated Solar PV planning. For validation, the ontology is used within the context of a district (about 1200 buildings) in the city of Vienna. This approach considers the perspectives of different stakeholders in the process. It aggregates the output results to a shared level of understanding among all stakeholders. It allows the integration of different computation models. Finally, it provides an extension possibility in case the ontology is to be used in a city with a different da-ta-availability.