We present the results for a new building supply and communication architecture based on Power Over Ethernet (PoE) with the IEEE 802.3bt standard. Driven by technological advances like photovoltaic production, LED lighting terminals, computer terminals, tablets, screens and the advancement of power electronics that allow more and more conversions with very high efficiency, direct current in buildings might be the right solution to reduce energy consumption and go towards positive energy buildings. With the new IEEE 802.3bt PoE standard which allows 99 W and with the increasing demand for new services in smart buildings and the emerging of IoT devices, we propose a communication and supply architecture completely based on the IP protocol. This paper presents feedback of the first deployment of a smart building totally powered by PoE devices, we analyse the power supply architecture and the application communication protocol that we have used which is MQTT (Message Queuing Telemetry Transport).