Home Energy Management Systems (HEMS), e.g. energy monitors, are intermediary products that can visualize, manage, and/or monitor the energy use of other products or whole households. HEMS increasingly receive attention for their role in energy conservation in households. A literature review and a case study examine the mid-term effectiveness (more than 4 months) of HEMS. The case study presents the results of a 15-month pilot with a domestic energy monitor in the Netherlands. It explores the extent to which participants manage to sustain their initial electricity savings over time, with a special focus on the development of habitual energy-saving behaviour. The results show that the initial savings in electricity consumption of 7.8% after 4 months could not be sustained in the medium- to long-term. A second finding is that certain groups of people seem more receptive to energy-saving interventions than others. These participants quickly develop new habits and exhibit larger savings than other participants. Obviously, a ‘one-size-fits-all’ approach for home energy monitors cannot be justified. For HEMS to be effective, a deeper understanding is needed that embraces social science, contextual factors, usability, and interaction design research.