Efficiency is a measure of return on investment. Every process - whether lighting a light bulb or running a car - requires some amount of energy "investment" to perform, but only a fraction of that energy actually goes on to do useful work like illuminating your room or moving passengers down a highway. The useful work performed divided by the original total energy investment (usually expressed as a percent) is the energy efficiency.
This white paper discusses in detail the results of recent efficient desktop computer research conducted by Ecova (Formerly Ecos) and EPRI, demonstrating savings of 40% to 70% over computers meeting today's ENERGY STAR v4.0 desktop requirements.