Improvement in this metric is organized in a hierarchy of four kinds of potential: technical, economic, achievable, and program. Robert Stavins and Adam Jaffe have described the inability of consumers and society to optimize this metric, despite its cost-benefit, as its namesake “gap.” Improvements in this metric counterintuitively cause increases in total usage according to the Jevons (“JEV-ins”) paradox, which is a form of the rebound effect. Improvements in this metric, described by the (*) IEA as the “first fuel,” may make upwards of 40 percent of progress toward net zero by 2050. (-5[1])For homes and buildings, this metric can be improved with continuous insulation, heat pumps, and products approved by the DoE’s Energy Star. For 10 points, name this metric defined as the amount of energy services provided per unit of energy consumed. ■END■ (10[1])