Are "green" products efficient? This exercise clarifies the meaning of efficiency to economics students; using data from window manufacturers, it contrasts the economics of buying a "high efficiency" window, which saves energy but costs more, with a normal window, which is cheaper but uses more energy. Students work together to brainstorm the logic of efficiency and present value, crunch some numbers, calculate which window is more efficient, and then speculate how changing interest rates changes the efficiencies involved. Includes grading rubric that explains the economics. Works either as an assigned paper or a discussion.