Energy efficiency has become a critical issue for battery-driven computers. Significant work has been devoted to improving it through better software and hardware. However, the human factors and user interfaces have often been ignored. Realizing their extreme importance, we devote this work to a comprehensive treatment of their role in determining and improving energy efficiency. We analyze the minimal energy requirements and overheads imposed by known human sensory/speed limits. We then characterize energy efficiency for state-of-the-art interfaces available on two commercial handheld computers. Based on the characterization, we offer a comparative study for them.Even with a perfect user interface, computers will still spend most of their time and energy waiting for user responses due to an increasingly large speed gap between users and computers in their interactions. Such a speed gap leads to a bottleneck in system energy efficiency. We propose a low-power low-cost cache device, to which the host computer can outsource simple tasks, as an interface solution to overcome the bottleneck. We discuss the design and prototype implementation of a low-power wireless wrist-watch for use as a cache device for interfacing.With this work, we wish to engender more interest in the mobile system design community in investigating the impact of user interfaces on system energy efficiency and to harvest the opportunities thus exposed.