The integration of volatile, decentralized energy into building energy systems and electrical grids requires intelligent algorithms that are capable of controlling the augmented complexity and the interactions of subsystems efficiently. Not only do the algorithms require computing capacity, but also a feasible information and communication infrastructure. One possibility to provide computing power on demand is cloud computing, either as a self-hosted or third party solution. Together with an extended connectivity of physical devices, it forms the Internet of Things (IoT). In this paper, we present a customizable open-source IoT platform setup using open source software, in particular FIWARE. For the integration of existing building automation networks, we extend an OpenMUC gateway with BACnet functionality and connect it to our platform via MQTT. We successfully demonstrate the overall setup with a web-based visualization of high resolution energy monitoring as basic requirements for future energy management systems.