TL;DR
- WUE = annual site water consumption (in litres) divided by annual IT energy (in kWh). Lower is better.
- Standardised as ISO/IEC 30134-9. Designed as a companion to PUE so that operators cannot simply trade water for energy and claim a sustainability win.
- Hyperscale facilities range from 0 L/kWh (fully dry-cooled) to 1.8+ L/kWh (heavy evaporative cooling). UK and EU averages cluster around 0.3-0.6.
- Increasingly used by regulators and planning authorities to assess new data centre water permits, especially in water-stressed regions.
Definition#
Water Usage Effectiveness is the data centre industry's primary water metric. ISO/IEC 30134-9 defines it as annual site water consumption (in litres) divided by annual IT energy consumption (in kilowatt-hours). The result is reported in L/kWh.
Site water means water consumed within the data centre boundary — typically evaporative cooling-tower losses, humidification, and any other on-site uses. It does not by default include the upstream water consumed to generate the electricity (that is the separate metric WUE-source).
Typical Values#
| Facility design | WUE (L/kWh) | Notes |
|---|---|---|
| Fully air-cooled (dry coolers) | 0.0-0.05 | Pure air rejection; no evaporative loop |
| Liquid-cooled with dry coolers | 0.0-0.10 | Free cooling in temperate climates, no evaporation |
| Adiabatic-assist cooling | 0.10-0.40 | Spray on hot days only |
| Evaporative cooling tower (closed loop) | 0.40-1.00 | Common in older hyperscale builds |
| Open evaporative tower (legacy) | 1.20-2.00 | Discouraged in water-stressed regions |
Why WUE Matters#
- PUE alone incentivises evaporative cooling — water is 'free energy' for heat rejection. WUE prevents that gaming.
- Water stress is now a planning constraint. Local authorities in water-stressed UK, EU, and US regions increasingly cap or reject new data centre permits that propose evaporative cooling.
- Net-zero programmes (including the EU Climate-Neutral Data Centre Pact and UK government procurement criteria) require both PUE and WUE targets.
- WUE-source — including upstream electricity water footprint — can be 4-5 L/kWh even for sites with WUE-site near zero, depending on the grid mix.
Trade-offs With PUE#
There is a clear trade-off between PUE and WUE. Evaporative cooling lowers PUE (warmer outdoor wet bulb = colder approach) but raises WUE. Dry coolers raise PUE in hot weather (mechanical cooling kicks in) but keep WUE near zero. Liquid cooling at warm-water temperatures (W3/W4) cuts both metrics simultaneously.
- Hot, humid climate + evaporative: low PUE, high WUE.
- Temperate climate + dry coolers + warm-water DLC: low PUE, near-zero WUE — the current best-practice combination.
- Cold climate + free air: low PUE, low WUE, but limited number of sites globally.
- Hyperscale air-cooled in hot climate: PUE around 1.3-1.4 with adiabatic spray on the worst days; WUE in the 0.2-0.4 range.
Operational Pitfalls#
- Make-up water vs blowdown: WUE counts net water consumed (make-up minus blowdown that is recycled). Some operators report only make-up, which overstates consumption.
- Humidification: in cold, dry climates, winter humidification can dominate WUE. Modern ASHRAE A1/A2 envelopes allow much wider humidity ranges (8-80 % RH), making humidification optional for most server classes.
- Reclaimed water: using treated greywater or industrial reclaim still counts toward WUE-site but materially improves the upstream picture and the planning narrative.
- Reporting boundary: confirm whether quoted WUE includes only the data hall or also office, lavatory, and landscaping water — the latter inflates the number without telling you anything useful about cooling.
References
- ISO/IEC 30134-9 — Water Usage Effectiveness · ISO
- The Green Grid — WUE White Paper · The Green Grid
- Climate Neutral Data Centre Pact · CNDCP