In recent times, there has been a significant growth in applications for battery powered portable electronics, as well as low power sensor networks. While sub-threshold circuit design approaches can reduce the power consumption significantly, a design operating at sub-threshold voltages is not necessarily optimal in terms of energy consumption. In this paper, we describe a technique to find the energy optimum VDD value for a design, and show that for minimum energy consumption, the circuit should be operated at VDD values which are above the NMOS threshold voltage value. We study this problem in the context of designing a circuit using a network of dynamic NOR-NOR PLAs.