There is broad agreement that the need to decarbonise and make better use of renewable and more intermittent sources of power will require increased flexibility in energy systems. However, organisations involved in the energy sector work with very different interpretations of what this might involve. In describing how the notion of flexibility is reified, commodified, and operationalised in sometimes disparate and sometimes connected ways, we show that matters of time and timing are routinely abstracted from the social practices and forms of provision on which the rhythms of supply and demand depend. We argue that these forms of abstraction have the ironic effect of stabilising interpretations of need and demand, and of limiting rather than enabling the emergence of new practices and patterns of demand alongside, and as part of, a radically decarbonised energy system. One way out of this impasse is to conceptualise flexibility as an emergent outcome of the sequencing and synchronisation of social practices. To do so requires a more integrated and historical account of how supply and demand constitute each other and how both are implicated in the temporal organisation of everyday life. It follows that efforts to promote flexibility in the energy sector need to look beyond systems of provision, price, technology, and demand-side management narrowly defined, and instead focus on the social rhythms and the timing of what people do.