The design of an energy-intensive system for lower cost and higher efficiency is one of the essential approaches to sustainable development. A principle is presented whereby exergy is elevated from a tool that reveals opportunities of higher efficiency to a tool that reveals opportunities of both lower cost and higher efficiency. The principle states that minimizing the individual costs of the energy conversion devices of a system with respect to their efficiency decision variables leads to a minimized cost of the system and an optimal overall system efficiency. The cost of a device is the sum of its fixed charges and its running expenses. The cost of exergy destruction of a device is its major running expense. The premises on which the principle is established are explained. Second-law analysis and design models of energy conversion devices play a central role in establishing these premises. Three useful design model manipulations seeking equations for the costing and for the performance of energy conversion devices are presented. Applications to the optimal design of systems and extensions to their optimal operation are given elsewhere.