W-Index

An index of water efficiency used as a device for evaluating residential water savings and as a management tool to motivate water-saving practices. The index provides a calculated numerical value for each dwelling unit, derived from the number and kind of water-saving features present, including indoor and outdoor water savers and water harvesting or recycling systems.


Source: Corrosion-Doctors.org

Wikipedia:  https://en.wikipedia.org/wiki/W_index