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 (15[1]-5[1])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 (-5[1])toward net zero by 2050. (10[1])For homes and buildings, this metric can be improved with continuous insulation, heat pumps, and products (10[1])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[2])