Much policy effort focuses on energy efficiency of technology, though not only efficiency but also user behaviour is an important factor influencing the amount of consumed energy. This paper explores to what extent energy efficiency of appliances and houses or user behaviour is the more important, both for understanding why some households consume much more energy than others, and when looking for relevant approaches to a future low-carbon society. The paper uses several sources to explore this question, most of them from a Danish context, including results from the researcher's own projects and Danish national statistics. These Danish data are discussed together with international studies. Through the presentation of these different projects and examples, it is shown how user behaviour is at least as important as the efficiency of technology when explaining households' energy consumption in Denmark. In the conclusion, these results are discussed in a broader international perspective and it is concluded that more research in this field is necessary. In relation to energy policy, it is argued that it is not a question of technology efficiency or behaviour, as both have to be included in future policy if energy demand is actually to be reduced. Furthermore, it is also argued that not only individual behaviour is relevant, but also a broader perspective on collectively shared low-carbon practices has to be promoted.