The post below is a reproduction of a paper published in the open access journal Sustainability by Michael Dale, Susan Krumdieck, and Pat Bodger (Vol. 3). The article is a first in creating a dynamic function where Energy Return on Energy Investment changes of an energy resource are estimated over time. In this manner it becomes possible to get an estimate of how much net energy a given fossil oil, gas, coal or renewable energy source yields during its lifetime. The created EROI function is based on theoretical considerations of energy technology development and resource depletion.