WUE (Water Usage Effectiveness) applies the logic of PUE to the water loop: it divides the water the data center consumes over a year, in litres, by the energy delivered to IT equipment, in kWh. It is expressed in L/kWh, and it was defined by The Green Grid, the consortium that published PUE.
The two figures are read together. Adiabatic or evaporative cooling lowers PUE by letting water evaporate: the site gains electrical efficiency by consuming water. Conversely, a closed loop or dry coolers use almost no water but draw more electricity. A very low PUE quoted with no water figure does not tell you the site is frugal, only where the cost was moved.
Two scopes exist, and they are rarely specified: site WUE counts only the building’s water, while source WUE adds the water needed to generate the electricity consumed. The second is always far higher, so ask which of the two you are being given.
Why this concerns you: during a drought, a water restriction can force the site onto a backup cooling mode that is less efficient. PUE then rises, and if your energy is re-billed at cost, your invoice rises with it. What to check: the measured annual figure, the scope it uses, where the water comes from (mains, borehole, reclaimed water) and the operating plan for a heat wave or a local water-restriction order.
Worth knowing: WUE is rarely published. We do not carry it in the catalog: if the operator does not publish it, we do not invent it. No answer to this question is itself a piece of information.