At first sight the purpose of energy efficiency is plain: it is to reduce the amount of energy used and the carbon emissions associated with the design and operation of things like buildings, domestic appliances, and heating and cooling technologies, or with the organization of bureaucratic, business or industrial processes. National and international responses to climate change are dominated by policies that promote energy efficiency and by people who take this to be a self-evidently important thing to do. Established criticisms, including those which focus on problems of rebound, draw attention to the unintended consequences of such strategies, but rarely challenge the conceptual foundations of ‘efficiency’ as a topic in its own right. This paper uses Bruno Latour’s We Have Never Been Modern (1993) notion of purification and Ian Hodder’s Entangled: An Archaeology of the Relationships Between Humans and Things (2012) ideas about entanglement to develop a more fundamental critique and to argue that, far from being a solution, efficiency, as currently constituted, undermines that which it is expected to achieve. It is concluded that if carbon emissions are to be reduced on any significant scale, then it is essential to consider the meanings and levels of service and the types of consumption and demand that efficiency policies support and perpetuate.