Picking time reduction has been the traditional perspective for warehouse optimisation. When sustainability is considered, optimisation of warehouse operations should be read in terms of energy efficiency other than response time. In an AS/RS it can be imagined to associate to each location in a rack the value of energy consumed by the crane to reach it, other than time. Since picking performance strictly depends on storage location assignments, a time based full turnover strategy is compared to an energy based one. Three models of energy consumption are considered for traditional and new generation cranes. Assignments are then compared in terms of dedicated zone shapes, time and energy performance within a given time horizon. Different shapes of the rack and product ABC curves are analysed.