Efficiency is a unit less physical quantity expressed in percentage and defined as the ratio of output energy over input energy. Since electrical energy is distributive, in some cases, the use of less efficient power generators is justified as long as abundant sources of cheap energy exist (e.g. coal, petroleum, natural gas, and other fossil fuels). Their demands have continuously increased since the Industrial Revolution of the late 18th and early 19th century. Unfortunately, they are of limited availability. Few nations have and many have not while supplies will eventually be depleted and beforehand would have already caused international economic instability. The future of generations facing this calamity is truly unthinkable. Optimistically, by then more reliable sources would be discovered. But the search for them must have already begun. Although renewable sources like solar and wind power proved promising, their intermittent (starts and stops) and storage problems made them that much more undesirable for widespread transmission but restricted local operations. But who can afford millions of dollars of installation costs of solar cells and wind fans in their backyards or the loss of environmental aesthetics? Moreover, there is always the lingering problem of achieving maximum efficiency.

The efficiency of an ideal Carnot heat engine is given by E=1-C/H where C is the absolute temperature of the heat sink and H that of the heat source. The lower C becomes and the higher H becomes the closer the efficiency to 100%. If the heat sink is at absolute zero then the efficiency is exactly 100%. On the contrary if C=H then the efficiency is zero. Outer space is 3 kelvins (CMBR threshold) while the sun has interior temperature of 10 millions kelvins. The calculated efficiency is 99.99997% and coincidentally the sun has been operating likely at constant rate for about 5 billion years. The only comparable heat output of this magnitude is that of high powered lasers. A heat engine made of laser irradiation of cryogenic deuterons will more than likely fulfill the limitless energy promise of cold fusion.