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Data center water use is most often summarized with Water Usage Effectiveness (WUE): the water a facility consumes on site in a year, divided by the energy its IT equipment uses, expressed in liters per kilowatt-hour (L/kWh). WUE is a useful way to compare cooling intensity between facilities, but it is not a total water figure, it leaves out the water used to generate the electricity a site draws, and it says nothing by itself about whether the local water supply can carry the load. A low WUE can sit alongside higher energy use, higher off-site water use, or a site in a drought-exposed basin. Real-world resilience comes from reading WUE next to cooling design, the workload served, and local water conditions.
What WUE measures
The U.S. Department of Energy’s Federal Energy Management Program (FEMP) defines site WUE as annual site water usage divided by IT-equipment energy, in liters per kWh, in its January 2019 guidance on cooling water efficiency for federal data centers. The July 2024 DOE Best Practices Guide for Energy-Efficient Data Center Design uses the same site definition.
Because the numerator is a volume and the denominator is energy, WUE measures intensity, not consumption. Two facilities can share the same WUE and still differ tenfold in total water. The figures below are illustrative arithmetic, not measured data:
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match| Facility (hypothetical) | Annual site water | Annual IT energy | Site WUE |
|---|---|---|---|
| A | 1,000,000 L | 2,000,000 kWh | 0.5 L/kWh |
| B | 10,000,000 L | 20,000,000 kWh | 0.5 L/kWh |
Ranking facilities by WUE alone therefore hides differences in scale.
What WUE leaves out
Water used to generate the electricity
Site WUE counts only water consumed at the property. Source WUE adds the water required to produce the electricity the facility uses, a distinction the DOE Best Practices Guide references. Source accounting is harder because it depends on the electricity supply, which depends on how the local grid generates power. A cooling change that cuts onsite water while raising electricity demand can improve site WUE and worsen the source picture.
Energy use
The Lawrence Berkeley National Laboratory’s 2024 United States Data Center Energy Usage Report makes the central trade-off explicit: air-cooled chillers use no water onsite but use more energy, while water-cooled and evaporation-based systems are generally more energy efficient and use more water. A WUE ranking would reward the air-cooled design for its water figure and ignore its energy penalty.
The workload and the hardware
WUE describes the building, not the computing inside it. A 2025 review by Nuoa Lei, Jun Lu, Arman Shehabi, and Eric R. Masanet, published in Resources, Conservation and Recycling (article 108310, DOI 10.1016/j.resconrec.2025.108310) and available from Berkeley Lab, assesses water use at the level of individual data center workloads. It reports variation of more than 10,000-fold in workload-level water use. The authors attribute that spread to two components: more than 1,000-fold variation in water consumed per kWh of server electricity, and roughly 10-fold variation in server workload efficiency. The determinants they identify also include server utilization, cooling type, infrastructure efficiency, climate zone, the share of inactive servers, and the server refresh cycle. These are the review’s findings, not a universal operating figure for any facility.
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How cooling water is consumed
In a cooling-tower arrangement described in DOE FEMP guidance, the process runs in four steps:
- IT equipment releases heat, which the facility’s cooling system carries away.
- The heat is rejected at the cooling tower.
- At the tower, water evaporates and carries heat to the atmosphere. This evaporated water is consumed on site.
- Evaporation leaves dissolved minerals behind in the water that remains. The system discharges blowdown to limit that buildup and adds makeup water to replace what has been lost.
Water use therefore follows the facility’s heat load and the efficiency of each stage of heat removal. Makeup and blowdown both count toward onsite consumption, so a measure that reduces either lowers the total.
Cooling choices compared
Cooling architecture moves the impact between onsite water and electricity. The table reflects the direction of effects reported in the LBNL 2024 report. The source-water column is a derived implication, not a separate measurement, so compute it with each site’s own electricity supply mix.
Rank #3
| Approach | Onsite water | Energy use | Source-water direction |
|---|---|---|---|
| Air-cooled chillers | None onsite | Higher | Rises with the additional electricity demand |
| Water-cooled and evaporation-based systems | Generally higher, through evaporation, makeup, and blowdown | Generally more energy efficient | Follows electricity demand, which is generally lower |
Operating measures that change the numbers
For existing cooling-tower systems, DOE FEMP lists operations and maintenance opportunities. Each depends on controls and operating conditions suited to the site and equipment:
- Review temperature and humidity setpoints.
- Use air-side economizing when outdoor conditions and air quality allow.
- Use water-side economizing when the system configuration permits.
- Optimize cycles of concentration to reduce blowdown and makeup water.
Cycles of concentration
Cycles of concentration is the ratio of dissolved solids in the circulating tower water to those in the makeup water. Raising it reduces how much water the tower must discharge as blowdown and draw as makeup. DOE FEMP, citing its Cooling Tower Best Management Practice, reports that increasing cycles from three to six reduces cooling-tower makeup-water requirements by 20% and blowdown by 50%. The guidance does not state the publication year of that practice. These percentages come from the guidance rather than from any specific facility’s records, so a site should verify them against its own water chemistry and operating data.
The trade-off is chemistry. Fewer blowdown events leave more dissolved minerals in the circulating water, so water treatment and control become more important as cycles rise.
Reverse-osmosis treatment of blowdown
DOE describes reverse-osmosis treatment of cooling-tower blowdown as one option to offset some freshwater needs in drought-affected regions. It produces a concentrated reject stream that must be managed, and DOE notes that hybrid systems add control loops and need a detailed operations and maintenance plan. It is an engineering option for specific sites, not a default recommendation.
A reported reference case
DOE FEMP also cites a National Renewable Energy Laboratory data center example with a PUE of 1.06 and a WUE of 0.7. It shows that a facility can report low energy overhead and low WUE together. The FEMP page does not give the year in which that case was measured, and a single reported facility is not a benchmark.
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WUE cannot score resilience because resilience depends on conditions outside the building. Assess each site on the following:
- Freshwater availability and drought exposure in the basin that supplies the facility.
- Utility constraints on the water supply serving the site.
- Operating controls: whether setpoints, economizing, and concentration cycles can actually run on the installed equipment.
- Energy implications: a water-saving cooling change that raises electricity demand shifts impact to the grid, and that impact depends on the supply mix.
- Climate: outdoor temperature and air conditions determine whether air-side or water-side economizing is practical.
- Workload and hardware: utilization, inactive servers, and the refresh cycle change water and energy use per unit of computing.
The published guidance does not provide a universal resilience score or locality-specific permitting rules. Water permitting and allocation vary by jurisdiction, so operators should confirm requirements with the local water utility and regulator.
National context
The most recent national update is Lawrence Berkeley National Laboratory’s United States Data Center Energy Usage Report: 2025 Update, published in June 2026. Its abstract estimates that data centers could account for 11.8% of U.S. electricity use by 2030, with scenarios ranging from 9.5% to 15.3%. Those are electricity estimates. The abstract does not establish a national water projection, so national water totals should not be inferred from it.
Quick Recap
Checking a site or design against the full picture
- Record site WUE together with the annual site water and IT energy behind it, so intensity is read alongside scale.
- Itemize onsite water: makeup, blowdown, and whether cycles of concentration are optimized.
- Estimate source water from the electricity demand difference between designs and the site’s supply mix.
- Compare energy use for each cooling approach. A design that saves onsite water but adds energy has shifted its impact rather than removed it.
- Test local water availability, drought exposure, and utility constraints against the design’s water demand.
- Confirm that the controls, maintenance plan, and staffing needed for any economizing or treatment option are in place.
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