A mobile energy harvester device based on the inertial automatic winding mechanism found in watches is explored. Through normal human motion during walking and running, the arm travels a spatial path that can potentially be used for energy harvesting. The conceptual harvester consists of a rotary pendulum coupled to a small generator through a step-up gear train. The generator’s electrical output may be stored and utilized as a power source for portable electronic devices that require a smaller amount of power for operation. In this paper, the equations of motion governing the human arm motion dynamics and harvester pendulum excitation are fully derived. Two cases of human walking and running are considered to analyze the system response. A series of representative simulation studies have been conducted for representative arm motion to determine the potential energy. The energy available for harvesting was greater in the case of the human subject running at 2.06 mJ, while when walking it offered an output of 0.42 mJ for a 5 second time period. The two numerical results serve as a basis for building a mobile energy harvester for future research into a renewable device that can be used by humans to augment battery life for portable electronic devices.