We consider a portable video device which captures and compresses video data. The compressed bit stream is either stored in an on-board storage device or transmitted to a remote user through wireless channels. The storage space or the amount of energy and channel bandwidth available for data transmission determines the total number of bits that the video encoder can generate. Besides this bit resource constraint, the portable video device also operates under data processing energy constraint. This is because video compression is computationally intensive and energy-consuming. One of the central challenging issues in portable video communication system design is how to maximize the operational lifetime. In this work, we will investigate the following research problem: given a portable device, which has a certain amount of power supply E and the maximum number of bits it can store or transmit is R, and the required video quality level is D, what is the maximum operational lifetime of this portable device, and how to allocate the bit and energy resources to achieve this maximum lifetime? To address this problem, we design an energy-scalable video encoder and study its power-rate-distortion (P-R-D) behavior. We theoretically study the optimum bit and energy resource allocation and analyze the energy saving performance. Our results show that for typical consumer electronic videos, using the proposed P-R-D video encoding technology, the operational lifetime of the portable device can be doubled or even tripled. This has a significant impact in energy-efficient portable video communication system design.