This paper describes a custom design approach as applied to power management in an innovative course on rapid prototyping of computer systems at Carnegie Mellon. We emphasize the importance of the choice of user interface modalities on power consumption of wearable computers. The paper identifies the major components of power consumption in a wearable computer, and evaluates their respective contributions to power consumption. We have quantified the power consumption of text-, graphics-, and speech-based interfaces, providing a guideline for the design of future wearable/mobile computer systems.