According to The Green Grid, a water use metric allows a data center manager to understand the effect water consumption has on the local electric grid. By using WUE in conjunction with power usage effectiveness and carbon usage effectiveness metrics, an organization can reduce energy use and, in effect, reduce the amount of water and electrical power needed to run the data center efficiently.
To calculate simple WUE, a data center manager divides the annual site water usage in liters by the IT equipment energy usage in kilowatt hours (Kwh). Water usage includes water used for cooling, regulating humidity and producing electricity on-site. IT equipment energy includes any power drawn by hardware used in the day-to-day functioning of the data center. More complex WUE calculations are available from The Green Grid's website.