In this paper we offer energy metric for evaluating the need for transformational technology in the context of resources management for sustainability. This metric measures the margin for improvement between the actual energy used and the theoretical minimum needed for the same task, irrespective of the technology considered. We first define the parameter space of sustainability metrics. Furthermore, we argue that, for any energy related sustainability study, the rate of change of the related process energy use (hence, ultimately at the lowest hierarchical level) has to be considered for a given task, not only for a given technology. Finally, we identify the margin between actual energy resources use and theoretical minimum. The paper offers, as an example, a study (at the process level) of a state-of-the-art manufacturing technology, broadly used in many applications, e.g., manufacturing of compact heat exchangers for the automotive industry. The margin between the theoretical minimum and the actual energy resources use for bonding an aluminium assembly by brazing is established. It is confirmed that the minimum theoretical energy resources needed, associated with a manufactured product, may be a number of orders of magnitude smaller than the actual energy resources used, irrespective of the technology implemented. (C) 2014 Elsevier Ltd. All rights reserved.