This work relates the potential energy savings to the energy profile of a circuit. These savings are obtained by using gate sizing and supply voltage optimization to minimize energy consumption subject to a delay constraint [1,2]. The sensitivity of energy to delay is derived from a linear delay model extended to multiple supplies. The optimizations are applied to a range of examples that span typical circuit topologies including inverter chains, SRAM decoders, and adders. At a delay of 20% larger than the minimum, energy savings of 40% to 70% are possible, indicating that achieving peak performance is expensive in terms of energy. The analysis is extended to register files, minimizing energy across pipeline stages, and optimal parallelism.