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Data center water use is most often expressed as water usage effectiveness (WUE): the annual onsite water use of a facility divided by the energy consumed by its IT equipment, in liters per kilowatt-hour (L/kWh). WUE is a useful way to compare cooling efficiency at one site. It does not include the water consumed to generate the electricity the site draws, and it says nothing by itself about whether local water supplies can absorb the demand. A low site WUE is one input to a judgment about water impact and resilience, not a verdict on either.
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 use, in liters per kWh. The definition appears in FEMP’s cooling-water efficiency guidance for federal data centers, dated January 9, 2019. DOE’s later Best Practices Guide for Energy-Efficient Data Center Design (July 2024) also defines site WUE.
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Two features of the definition shape how the number should be read.
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- The denominator is IT energy only. Cooling-plant and power-delivery energy do not enter the ratio, so WUE cannot show whether the facility is efficient in its energy use. It has to be read beside an energy metric.
What WUE leaves out
Site WUE covers water used on the property. It excludes water consumed upstream at power plants to generate the facility’s electricity. The Lawrence Berkeley National Laboratory (LBNL) 2024 United States Data Center Energy Usage Report distinguishes site WUE from source WUE, which accounts for the water required to produce the electricity the data center uses.
Source accounting is harder than site accounting because it depends on the electricity supply. The water intensity of a grid varies by region and by generation mix, so two facilities with the same site WUE can carry very different source-water footprints. Any comparison that involves a change in electricity demand needs to state which boundary it uses.
How cooling design moves the impact
Cooling choice changes which resource is affected. The 2024 LBNL report notes that air-cooled chillers use no water onsite but use more energy, while water-cooled and evaporation-based systems generally can be more energy efficient and use more water. The trade is between onsite water and energy, and each side has its own source-water consequence.
| Cooling approach | Onsite water | Energy use | Source-water effect |
|---|---|---|---|
| Air-cooled chillers | None onsite (LBNL 2024) | Higher (LBNL 2024) | Not quantified in the cited sources; depends on the electricity supply |
| Water-cooled and evaporation-based systems | Uses onsite water; more than air-cooled chillers (LBNL 2024) | Generally more energy efficient (LBNL 2024) | Not quantified in the cited sources; depends on the electricity supply |
Where onsite water goes
In a cooling-tower arrangement, heat from IT equipment is transferred through the cooling system and rejected at the tower, where evaporation carries it to the atmosphere. Water use therefore tracks the facility’s heat load and the efficiency of each stage of heat removal, as described in FEMP’s cooling-water guidance.
Evaporation is only part of the onsite water budget. As water evaporates, dissolved minerals stay behind and concentrate in the circulating water. Operators discharge some of that concentrated water as blowdown and replace it with makeup water. Makeup water therefore covers both evaporation and blowdown, which is why a cooling tower’s makeup demand is larger than the water that evaporates alone.
Rank #3
Off-site water in the electricity supply
A 2025 review by Nuoa Lei, Jun Lu, Arman Shehabi, and Eric R. Masanet, “The water use of data center workloads: A review and assessment of key determinants,” in Resources, Conservation and Recycling 219 (2025), article 108310 (DOI 10.1016/j.resconrec.2025.108310), assesses water use at the level of individual workloads rather than whole sites. The review ranks the following as key determinants:
- server efficiency
- grid water-consumption factors
- server utilization
- cooling type
- infrastructure efficiency
- climate zone
- share of inactive servers
- server refresh cycle
The review reports workload-level water use varying by more than 10,000-fold. That spread is driven by two factors it quantifies separately: water consumption per kWh of server electricity varies by more than 1,000-fold, and server workload efficiency varies by roughly tenfold. These are the review’s findings across the workloads it assessed. They are not operating measurements of any particular facility and should not be used as a benchmark for one.
Electricity demand is the channel through which this off-site water grows. LBNL’s United States Data Center Energy Usage Report: 2025 Update, published in June 2026, estimates in its abstract that data centers could account for 11.8% of U.S. electricity use by 2030, with scenarios from 9.5% to 15.3%. Those are electricity estimates. The abstract does not project national water use, so the figures should not be read as a national water number.
Operational measures that lower onsite water
FEMP’s guidance lists operations and maintenance opportunities for existing cooling-tower systems. DOE states that these can lower water use in appropriate systems and climates, and each depends on controls and operating conditions suited to the site and equipment:
- Setpoint review. Reviewing temperature and humidity setpoints.
- Air-side economizing. Using outdoor air for cooling when outdoor conditions and air quality allow.
- Water-side economizing. Using water-side economizing when the system configuration permits it.
- Cycles of concentration. Optimizing cycles of concentration to reduce blowdown and makeup water.
Cycles of concentration: the three-to-six example
Cycles of concentration is the ratio of dissolved minerals in the circulating tower water to those in the makeup water. Raising it means less blowdown for each unit of evaporation, but the circulating water is then more concentrated, so water treatment has to keep scaling and corrosion under control.
FEMP cites its Cooling Tower Best Management Practice for one example: increasing cycles of concentration from three to six reduces cooling-tower makeup-water requirements by 20% and blowdown by 50%. The FEMP page does not give a publication year for that underlying practice, and the percentages describe that example, not every tower.
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. The process produces a concentrated reject stream. DOE also notes that hybrid systems add control loops and need a detailed operations and maintenance plan. It is an engineering option to evaluate for a specific site, not a default measure.
Why resilience depends on the site
The same cooling design can be sound in one location and a poor fit in another. Resilience depends on local freshwater availability, drought conditions, and utility constraints. Climate and outdoor-air conditions determine whether economizing is possible at all. Operating controls matter as well: an economizer or cycles-of-concentration strategy saves water only when the equipment and its controls support it.
Changing the cooling approach also changes the energy side of the ledger. Moving from water-based to air-based cooling trades onsite water for energy that still has to be supplied, which can shift the burden onto the grid and its water. No public score converts these factors into a single resilience rating, and the guidance cited here does not replace a local water-utility or permitting review.
Quick Recap
A checklist for judging a data center water claim
- Confirm the boundary. Establish whether the figure is site WUE (onsite water only) or includes source water. A comparison is valid only when both figures use the same boundary.
- Check the denominator. Confirm that WUE is divided by IT-equipment energy, not total facility energy.
- Note the measurement year. Guidance and case examples carry different dates. FEMP’s NREL data-center example, which reports PUE 1.06 and WUE 0.7, does not state the year in which it was measured.
- Identify the cooling design. Air-cooled, water-cooled, and evaporative designs imply different water-energy trade-offs, as shown in the table above.
- Ask for the energy figure beside the water figure. A water number on its own cannot show whether the trade was worthwhile.
- Check the workload behind the figure. Use the determinants listed above, particularly utilization, inactive-server share, and server efficiency, to judge what the water is producing.
- Test the figure against local conditions. Apply the freshwater, drought, and utility factors described above to the specific location before deciding whether the figure is acceptable.
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