"natural resource for which the ratio of the creation of the natural resource to the output of that resource from nature to the technosphere is equal to or greater than one". Examples During the 1920s, of crude oil were extracted for every barrel of crude used in the extraction and refining process. Today only are harvested for every barrel used. When the net energy gain of an energy source reaches zero, then the source is no longer contributing energy to an economy. See also ISO 13600 Energy balance Energy returned on energy invested Energyware and energy carrier Solar cells and energy payback Energy cannibalism References External links ISO 13602-1:2002 Methods for analysis of technical energy systems. The Importance of ISO and IEC International Energy Standards. Technical energy systems Thinking clearly about biofuels: ending the irrelevant net energy debate and developing better performance metrics for alternative fuels. Energy economics