Energy consumption increases in all end-use demand sectors, but in the buildings sector it grows fastest throughout the projection. In this sector, the recent energy sustainability polices has strongly addressed energy efficiency strategies in order to stabilize or reduce the energy demand growth, because recognized as essential and cost-effective ones. Stakeholders and scientists have recognized wireless sensor networks (WSN) as challenging and enabling technology for achieving energy efficiency in buildings, allowing to individuals to monitor, control the electricity demand of all appliances in the households or offices as well as of computing hardware and related infrastructures in data centers (DCs). More recent energy-aware systems, WSN-based, are essentially based on continued and timely monitoring approaches of the power usage by the electrical appliances in a building, standing out for the type of sensors they use or the spatial granularity used for collecting data. An issue that these energy-aware systems have not taken into account, and which the system and the method proposed in this research work addresses, is that monitoring buildings' air quality, safety and environmental conditions coupled with energy power usage enables more effective energy savings strategies in buildings. The sensing system and method here presented for achieving energy efficiency in the building sector is based on wireless multi-sensor modules provided with on-board intelligence, capable of obtaining real-time information on energy consumption, quality of the air, environmental variables, and occupation of the premises. These intelligent sensing devices are organized in a multilevel architecture of intelligent sensor network to enable continued, pervasive and distributed monitoring aimed at making the user more aware of energy consumptions and enabling active control polices for energy-consuming systems, in order to optimize the power usage.