Multi-mode systems are characterised by a set of interacting operational modes to support different functionalities and standards. In this paper, we present a co-design methodology for multi-mode embedded systems that produces energy-efficient implementations. Based on the key observation that operational modes are executed with different probabilities, i.e., the system spends uneven amounts of time in the different modes, we develop a novel co-design technique that exploits this property to significantly reduce energy dissipation. We conduct several experiments, including a smart phone real-life example, that demonstrate the effectiveness of our approach. Reductions in power consumption of up to 64% are reported.