Compare data centre cooling systems by their annual energy and water performance under the same workload, climate assumptions and measurement boundaries—not by technology label alone. PUE and WUE are useful starting points, but neither captures every trade-off in IT efficiency, resilience or water impact. The right choice depends on site conditions, equipment heat density, water availability and the facility’s ability to operate and maintain the system.
Which metrics make a fair comparison?
Ask for both energy and water results, and establish how each figure was measured before comparing systems. A low value in one metric does not guarantee a strong result in the other.
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- PUE (Power Usage Effectiveness) is annual total facility energy divided by annual IT equipment energy. It indicates how much facility energy is used relative to the IT load; it does not measure the useful computing work performed or water consumption. A ratio of 1.0 is the theoretical minimum, not an expected result for a particular cooling design.
- WUE (Water Usage Effectiveness) is annual site water use in litres divided by IT equipment energy in kWh in the definitions used by the U.S. Department of Energy and ASHRAE. Request the units and the water boundary: the figure is only comparable if both proposals count water in the same way.
For either metric, establish whether the figure is measured operating data or a modeled estimate, the period it covers, the IT load during that period, and the facility boundary. Report annual results separately from seasonal or point-in-time measurements. ASHRAE’s 2023 handbook notes that PUE is impractical as a projected design-stage efficiency measure, so label a design target as a projection rather than an observed result.
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Site WUE does not necessarily include water associated with generating the electricity the facility consumes. If that broader impact matters, request it as a separate assessment and define its calculation boundary; the two quantities should not be combined without a consistent method.
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How do the main cooling approaches compare?
These system families differ in how they move heat and when they need mechanical cooling. Actual performance depends on design, configuration and operating conditions, so use the comparison to identify questions for a site-specific proposal—not to assume a guaranteed outcome.
| Approach | Energy considerations | Site-water considerations | Key questions |
|---|---|---|---|
| Chiller and cooling tower | A conventional arrangement moves heat from IT equipment to room air, through air-handling equipment and chilled water to a chiller, and then through a condenser-water loop to a cooling tower. The energy required depends on the complete heat-removal chain and operating conditions. | Evaporative heat rejection consumes water. DOE notes that cooling-tower water consumption depends on IT and other facility heat loads and on the efficiency of the heat-removal chain. | What are the annual measured PUE and WUE at the proposed load? How much water does the tower use across seasons? |
| Water-side economizing | A heat exchanger can use cooling-tower water to cool the chilled-water loop in suitable outdoor conditions, reducing or bypassing chiller compressor operation. Performance depends on outdoor conditions and system configuration; an upstream, series heat exchanger can provide a first cooling stage. | Because the approach uses cooling-tower water, its water implications depend on the tower and operating mode. Economizing does not by itself establish low WUE. | Is an integrated heat exchanger installed, and when does the system actually bypass the chiller? What are the seasonal energy and water results? |
| Air-side economizing | When outdoor conditions are suitable, a direct air economizer brings outside air into the data hall and exhausts a similar volume of warm air, potentially reducing mechanical cooling. | The cited sources do not establish a universal water result for this approach; request a site-specific WUE and its boundary. | What outdoor conditions define the operating envelope, and what equipment requirements apply at this site? |
| Dry or adiabatic heat rejection | Dry heat rejection avoids routine evaporative cooling-tower operation. Adiabatic assistance can change performance in hot conditions, and energy use still depends on design and operating conditions. | Dry systems avoid routine evaporative water use for heat rejection; adiabatic assistance can consume water in hot conditions. | How often is adiabatic assistance used, and what are measured seasonal PUE and WUE? |
| Direct liquid cooling | Liquid transfers heat from IT equipment into a recirculating loop rather than relying first on room air. The loop may still connect to chilled water, a cooling tower or air cooling for other loads. | Water use depends on the heat-rejection equipment and configuration beyond the IT liquid loop; the label “liquid cooling” alone does not establish WUE. | Which loads are liquid-cooled, what remains air-cooled, and how are additional control loops commissioned and maintained? |
In an April 24, 2026 briefing, Uptime Institute Intelligence reported that well-designed dry and adiabatic systems could match evaporative-cooling PUE across ASHRAE climate zones 2–6 with zero or near-zero water consumption. Treat that as a finding from the briefing’s analyzed operating data, not a guarantee for every facility. The briefing highlights configuration, free-cooling capture and operational discipline as important determinants.
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What should you compare beyond the system label?
Use the same assumptions and reporting boundary for each candidate system. Ask vendors or design teams to provide comparable answers on the following dimensions:
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| Dimension | What to request | Why it changes the decision |
|---|---|---|
| Facility energy | Annual PUE, measurement boundary and period, IT load, and whether the value is measured or modeled. | PUE describes facility overhead relative to IT energy, not IT productivity or water use. |
| Water | Annual site water use and WUE in stated units, with the included sources, boundary and seasonal context. | Evaporative heat rejection may reduce energy use while consuming water; site water alone may omit water associated with electricity generation. |
| Climate and free cooling | Expected hours and conditions for air-side or water-side economizing, design temperatures and seasonal performance. | Outdoor conditions affect how often chillers or evaporative assistance are needed. |
| IT load and compatibility | Rack density, equipment thermal limits, air or liquid interfaces, and workload profile. | High-density equipment can change which heat-transfer approaches are feasible. |
| Reliability | Redundancy, failure modes, switchover behavior and operating envelope. | An efficiency comparison is incomplete if candidates do not meet the same service requirements. |
| Operations | Controls, water treatment, maintenance, staffing, commissioning and monitoring needs. | More operating modes can create optimization opportunities but require dependable controls and maintenance. |
| Broader impacts | Electricity source, local water stress and heat reuse, where relevant. | PUE and WUE do not represent every environmental or business impact. |
How should you assess economizers and liquid cooling?
Check what “free cooling” means in the design
Economizers can reduce compressor use when outdoor conditions permit, but the label does not tell you whether the system can capture that opportunity. For water-side economizing, check for an integrated heat exchanger and ask when it can bypass the chiller. For air-side economizing, confirm the site’s operating envelope and equipment requirements. Compare the expected operating hours and seasonal results rather than assuming every tower or air system has the same capability.
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Trace liquid cooling through the whole facility
Direct liquid cooling moves heat from IT equipment into a recirculating liquid loop, often using coolant distribution units. It can reduce reliance on room air to transport IT heat, but the liquid loop does not necessarily eliminate chilled water, cooling towers or air cooling elsewhere in the facility. DOE describes a hybrid thermosyphon case that combines liquid cooling with air-cooled heat rejection and an open tower, switching modes with outdoor conditions. Additional control loops make an operations and maintenance plan important.
How should you use published performance examples?
A reported result can show what one facility achieved, but it is not a general benchmark unless the workload, climate, system boundary and operating conditions are comparable. The U.S. Department of Energy Federal Energy Management Program’s 2019 page reports that the National Laboratory of the Rockies data centre achieved PUE 1.06 and WUE 0.7. These are results for that named facility, not a typical or guaranteed outcome; the reported information does not provide enough detail to normalize the figures for comparison with arbitrary sites.
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What should a cooling-system proposal include?
Before selecting an architecture, request a side-by-side comparison using a common annual period and load assumptions. Keep measured operating results separate from modeled design estimates.
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- Set the comparison basis. Specify the expected IT load and workload profile, the site climate and the required reliability level.
- Request the metrics with boundaries. Ask for annual PUE, annual WUE in litres per kWh, site water use, measurement boundaries, seasonal context and whether each value is measured or modeled.
- Ask for operating modes. Identify when chillers, air-side or water-side economizers, dry heat rejection and adiabatic assistance operate, and what conditions trigger a changeover.
- Check equipment fit. Document rack density, thermal limits and which IT or facility loads use air, liquid or both.
- Review resilience and operations. Request redundancy and switchover details, control and monitoring requirements, water treatment, maintenance plans, staffing needs and commissioning responsibilities.
- Compare broader impacts where needed. If electricity-related water use, local water stress or heat reuse matters, define how each will be assessed rather than assuming PUE or WUE covers it.
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