Energy efficiency is a central target for energy policy and a keystone for sustainable development. However, the lack of a valid set of energy efficiency indicators (EEI) sufficiently accurate, robust and repeatable is significantly affecting the success of such policies and causing no little confusion and speculation on this field. Therefore, it appears to be essential to identify suitable methods to measure energy efficiency and to achieve consensus over the use of a valid set of EEI. This paper reviews the definition of efficiency and its quantification. First, a methodology is introduced in order to construct EEI. This methodology is applied to heating, ventilation and air conditioning (HVAC) systems and an exhaustive classification of EEI is proposed at four levels: global, service, subsystem and equipment. A new 'subsystems approach' is introduced for the energy analysis of HVAC systems based on generation efficiency, transportation specific consumption and the 'demand ratio', a new measure of HVAC thermal energy efficiency. Finally, we present a case study to illustrate the application of the proposed EEI to HVAC systems and to show the main advantages of the 'subsystems approach'. (C) 2012 Elsevier B.V. All rights reserved.