This paper presents a case study for the evaluation of the impact of the Building Automation and Control Systems (BACS) on the energy performance of buildings. The “Edifício das Tágides”, a project for a four star hotel located in Paço de Arcos (Portugal) is analysed. The hotel has a useful area of 6832 m2 and a contemporary architecture. As a central element for the conception of the aforementioned systems, the BACnet (Building Automation and Control Networks) protocol was used. The BACnet is a powerful and flexible standardized protocol with recognized interoperability. It acts in the technical facilities that have greater impact on the building energy consumption: lighting, heating, ventilation and air-conditioning. The impact of the BACS in the energy performance of the building was determined according to two methods: a detailed-direct approach, and a simplified approach, both presented in the European standard EN 15232-Energy Performance of Buildings-Impact of Building Automation. The paper provides the motivation, the legal framework and a description of the proposed system, from design to implementation, including simulation results for the impacts in the building energy consumption and in greenhouse gases emissions. Results show that, the detailed method reports higher reductions, while the simplified method is significantly more conservative. Both methods confirm that the implementation of the automation systems leads to a significant reduction in the building energy consumption (39.7% and 17.7%, respectively) and in greenhouse gases emissions (38.3% and 16.7%, respectively).