Data from sensors on board the Polar, Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), and other Earth-orbiting spacecraft show that high-energy electron intensities vary in a global, coherent way throughout the outer trapping zone of the terrestrial magnetosphere. Long-term measurements of very energetic trapped electrons provide a comprehensive view of the large variability of intensities on timescales less than 1 day to times corresponding to the 11-year sun-spot cycle. The average density of relativistic electrons within the outer magnetospheric trapping region (2.5 is calculated in this paper during a 7-year observing period. The total energy content and power dissipation in this relativistic electron component are then compared on a yearly basis to the solar wind energy that impacts the magnetosphere during the same time. The overall efficiency of relativistic electron production is thereby computed and is found consistently to be ~1% throughout the entire observing interval. Such efficiencies are important and potentially useful in scaling electron acceleration in other, more remote solar, planetary, and cosmic systems. The results presented here suggest that a range of different processes are involved in producing very energetic electrons in various astrophysical plasmas. ¿ 2001 American Geophysical Union