This paper describes a system level design approach to power awareness in the wearable computers project at Carnegie Mellon University. The paper identifies the major components of power consumption in a mobile computer, evaluates their respective contributions to power consumption, and analyzes various techniques for improving their energy efficiency. The paper describes our research framework and experimental evaluations of techniques for improving energy efficiency of a system, ranging from the communication level down to the physical level of the battery. The work described includes techniques for dynamically varying the CPU clock frequency.