WUE — The Standard Measure
U.S. Department of Energy (FEMP / Better Buildings) data center guidance
Water Usage Effectiveness is the accepted metric for data center water performance: annual site water use divided by annual IT equipment energy, expressed in litres per kilowatt-hour.
What the metric says
WUE = annual site water use (L) ÷ IT equipment energy (kWh). A lower number means less water per unit of computing work. A fully air-cooled or sealed closed-loop site can report a WUE at or near zero for cooling, while an evaporative site reports a substantially higher figure.
Federal guidance
U.S. Department of Energy data center energy and water efficiency guidance (Federal Energy Management Program and the Better Buildings data center programme) ties data center water consumption directly to cooling and heat load, and directs operators to track WUE alongside PUE, to raise supply-air and chilled-water temperatures where equipment allows, and to consider alternative or reclaimed water sources. Government source, ongoing programme guidance.
Limits of the metric
WUE measures onsite water only. It says nothing about indirect water in the electricity supply, nothing about whether the water was potable or reclaimed, and nothing about whether the local basin could spare it. A good WUE in a stressed aquifer is not the same result as the same WUE beside an abundant supply.