Nate Hagens defines 'energy return' as the crucial measure of the energy yield obtained from an energy-investment process relative to the energy expended to facilitate it. Viewing energy as the bedrock of civilization, Hagens underscores that all societal functions—be it transportation, food production, or technological innovation—are contingent upon the surplus energy harnessed. He emphasizes that the concept of Energy Return on Investment (EROI) is pivotal in evaluating the viability and sustainability of energy sources. Higher EROI values indicate more efficient energy systems that can support complex societal structures, whereas lower EROI values suggest that more energy is required to procure energy, thereby straining economic systems and threatening long-term sustainability. Understanding energy return, according to Hagens, is vital for navigating future energy transitions amidst declining net energy availability.

See also: fossil fuel, renewable energy, nuclear power, solar panel, wind turbine