In this paper, we present a universal figure of merit, the Consumption Factor (CF), that may be used to judge the power efficiency of any communications device or link. We derive expressions for the CF that account for the power efficiency of components in a homodyne transmitter, as this transmitter architecture will typify many future mm-Wave and massively broadband systems . The CF is defined as the maximum ratio of data rate to consumed power as a function of the transceiver subsystem and channel parameters, and offers a way for circuit designers and communication engineers to optimize and compare designs of different radios. We show also how CF may be extended to other transmitter architectures beyond the homodyne architecture. As green radios and higher efficiency wireless devices become critical for the reduction of wasted power and improved battery life in very broadband communication systems, the analysis presented here gives a mathematical framework from which comparison and optimization of transmitter-receiver designs can be performed. This paper also provides a parametric study that shows how the analysis can be used to optimize spectral efficiency, and how to adjust an operating link set point for an operational signal-noise-ratio that achieves optimum data rates for a given consumed power level. The new methodology shows how the efficiencies and gains of different components in a radio circuit impact the CF, including antennas, power amplifiers, and mixers. The results show that the efficiency of components on the signal path of a transmitter closest to the point at which information is transmitted, such as the antenna, have the greatest impact on the CF. A follow-on study will show the impact of receiver components and channel conditions on the CF.