The mitigation of emission from shipping will require improvements in energy efficiency. In order to achieve this, sociotechnical changes are required, affecting all stakeholders within the shipping sector. Ship crews and their everyday work practices will play an especially important role in the transformation of the sector. It is therefore crucial to understand how new energy efficient technologies and practices are being introduced and enacted onboard ships. The case study reported in this paper investigates an attempt to improve the operational energy efficiency in a shipping company that was made by installing an energy monitoring system and introducing an energy saving policy onboard the ships in the fleet. The analytical framework in this paper is inspired by cultural-historical activity theory which is suggested as a novel and useful practice-based approach in energy studies. It is used in analyzing the contradictions and tensions in the work practices onboard the ships that preceded and followed the implementation of the energy monitoring system and energy saving policy. The empirical results revealed how the initial demand for operational energy efficiency and the subsequent introduction of the new monitoring system and policy gave rise to tensions in the existing activity systems onboard which crew members then tried, but did not always manage, to reconcile. It is concluded that a better understanding of the sociotechnical change processes, associated with organizational energy conservation and energy management, can be achieved if the situated paradoxes of practitioners’ everyday practices are examined.