Today, it is widely accepted, that energy consumption is becoming the first item of expenditure for network providers. Energy saving and carbon footprint limitations are becoming of high interest in the telecommunication sector. In this paper, we consider interface selection for wireless networks' access. We propose an Energy Consumption Optimization for Wireless Interface Selection Mechanism (ECO-WISM) that optimizes interface selection regarding QoS and energy consumption for mobile terminals that are equipped with multiple wireless network interfaces. Selection rules are set for sending and receiving activities for different wireless technologies and according to traffic requirements. Other important parameters are not neglected including the energy associated to the scanning and association phases. Performance evaluations of the proposed mechanism show that less energy can be consumed if appropriate decisions are made. These results are obtained without QoS degradations. The proposed scheme can reduce the overall energy consumption and lengthen mobile terminals' lifetime