The Tool Desk
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Which metrics should you compare?
Ask for annual operational measurements as well as the conditions behind them. Keep measured results separate from design-stage projections, and use the same reporting period and load assumptions when comparing facilities or designs.
| Metric or information | What to request | What it tells you—and what it does not |
|---|---|---|
| Power Usage Effectiveness (PUE) | Annual total facility energy divided by annual IT equipment energy; include the measurement boundary, reporting period, IT load and whether the figure is measured or modelled. | Shows facility energy overhead relative to IT equipment energy. It does not measure water use, the useful work performed by IT equipment or resilience. |
| Water Usage Effectiveness (WUE) | Annual site water use divided by annual IT equipment energy, commonly reported in litres per kilowatt-hour (L/kWh). State which water uses and facility areas are included, and provide seasonal context. | Relates site water consumption to IT energy. Results are not comparable if their water boundaries differ; site WUE also does not, by itself, account for water used to generate electricity. |
| Operating conditions | IT load and rack density, weather conditions, cooling modes in use, and any seasonal or point-in-time limits on the figures. | Helps explain when and why a system’s measured energy or water use changes. |
| Reliability and operations | Redundancy, failure modes, switchover behaviour, operating envelope, controls, water treatment, maintenance and monitoring requirements. | Shows whether an efficiency result is achievable while meeting the facility’s service and operational needs. |
PUE is a ratio, not a complete sustainability score. Efficient facilities can approach its theoretical minimum of 1.0, but a lower value alone does not establish greater IT efficiency, lower water impact or better resilience. WUE also needs context: a site-water-only comparison may omit water associated with electricity generation. If that broader impact matters, define a consistent boundary for it rather than combining unlike figures.
For design-stage comparisons, label PUE projections as estimates, not observed performance. ASHRAE’s 2023 guidance cautions that PUE is impractical as a projected design-stage efficiency measure. Report operational measurements separately and identify whether they cover a full year, a season or a particular moment.
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How do the main cooling approaches differ?
Compare actual configurations, not broad labels. Systems can combine several heat-transfer and heat-rejection methods, and a facility may use different modes as outdoor conditions change.
| Approach | How it handles heat | Energy and water considerations | What to check |
|---|---|---|---|
| Chiller with cooling tower | Room air carries heat from IT equipment to cooling equipment; chilled water transfers it to a chiller and condenser-water loop, which rejects heat through a cooling tower. | Evaporation in the tower consumes water. Total cooling-tower water use depends on IT and other facility heat loads and on the efficiency of the heat-removal chain. | Request measured annual energy and site water use, and establish which loads and cooling equipment are inside each measurement boundary. |
| Water-side economizer | A heat exchanger uses cooling-tower water to cool the chilled-water loop during suitable outdoor conditions, reducing or bypassing chiller compressor operation. | Can reduce compressor use when conditions and configuration allow. It does not guarantee the same savings at every site or in every tower-and-chiller arrangement. | Ask whether an integrated heat exchanger is installed, how it is arranged, what conditions enable economizing and how often the mode operates. |
| Air-side economizer | Brings suitable outdoor air into the data hall and exhausts a similar amount of warm air. | Can reduce mechanical cooling when outdoor conditions fall within the system’s operating envelope. | Check site conditions, equipment requirements, the allowable operating envelope and the hours when the mode can be used. |
| Dry heat rejection | Rejects heat without routine evaporative water use. | Avoids routine evaporative water consumption for heat rejection. Energy performance depends on design, climate, configuration and operation. | Compare site-specific annual energy and water measurements, not the technology label alone. |
| Adiabatic heat rejection | Uses dry heat rejection with water-assisted cooling in conditions when assistance is needed. | Water use can rise in hot conditions. Uptime Institute’s April 24, 2026 briefing reports that well-designed dry and adiabatic systems can match evaporative-cooling PUE across ASHRAE climate zones 2–6 with zero or near-zero water consumption in the analysed operating data; this is not a guarantee for every design. | Establish when water assistance is used, how performance changes by season and whether the reported result applies to the proposed configuration. |
| Direct liquid cooling | Transfers heat from IT equipment into a recirculating liquid loop instead of relying first on room air to carry it away. | Can reduce dependence on room-air heat transport. The liquid loop may still rely on chilled water, cooling towers or air cooling for heat rejection or other facility loads. | Check equipment compatibility, liquid-to-facility interfaces, operating modes, added control loops and the maintenance plan. |
Read system performance in context
Water-side economizing illustrates why the exact arrangement matters. The U.S. Department of Energy’s Federal Energy Management Program describes an integrated heat exchanger that can bypass the chiller and use the cooling tower to cool the chilled-water loop during mild outdoor conditions. DOE notes that placing the exchanger in series and upstream of chillers can make it a first cooling stage. A cooling tower alone does not establish that this economizer arrangement is present.
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Direct liquid cooling also describes a heat-transfer approach, not one complete facility design. A coolant distribution unit can serve a liquid loop while other loads still use air or chilled water. DOE’s hybrid thermosyphon example combines liquid cooling with air-cooled heat rejection and an open tower, changing modes with outdoor conditions. Such additional control loops need an operations and maintenance plan.
How should you compare two proposals?
Put both options against the same site conditions and service requirements. If the proposals assume different loads, weather data or reliability levels, first identify those differences; otherwise, the headline figures may not describe a like-for-like comparison.
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- Set a common basis. Specify the same annual period, IT load and workload assumptions. Identify what facility equipment and water uses are included in each measurement.
- Separate evidence types. Label annual measured operating data, seasonal data, point-in-time readings and modelled design estimates distinctly. Do not present a projection as an observed result.
- Test climate and economizer availability. Request the outdoor conditions and hours under which air-side or water-side economizing can operate, and the seasonal performance when those conditions do not apply.
- Match the design to the IT load. Compare rack density, equipment thermal limits, air or liquid interfaces and workload profile. These determine which heat-transfer arrangements are feasible.
- Account for water constraints. Compare site water use and WUE with the water source, boundary and seasonal variation stated. Consider local water stress and, where relevant, water associated with electricity generation under a separately defined boundary.
- Check resilience and operating capability. Compare redundancy, failure modes, switchover behaviour and operating envelope alongside controls, commissioning, monitoring, water treatment, staffing and maintenance needs.
- Assess other relevant impacts. Where they matter to the decision, include electricity source and heat reuse; PUE and WUE do not capture every environmental or business consequence.
These checks help distinguish a promising design feature from demonstrated site performance. They also expose trade-offs: for example, a system may reduce compressor energy in suitable weather while requiring water, or reduce routine evaporative water use while presenting a different energy profile in the site’s climate.
How much weight should you give published examples?
Published results can show what a particular facility achieved, but they are not interchangeable benchmarks unless their load, climate, system boundary and measurement period are comparable.
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DOE’s Federal Energy Management Program reported a PUE of 1.06 and a WUE of 0.7 for the National Laboratory of the Rockies data centre in a 2019 case. Those are results for that named facility, not typical or guaranteed outcomes for another site. The reported figures are not enough to normalise the case against arbitrary facilities.
Uptime Institute’s April 2026 briefing reports that analysed dry and adiabatic systems could match evaporative-cooling PUE across ASHRAE climate zones 2–6 with zero or near-zero water consumption. The finding is conditional: the briefing identifies design, configuration, free-cooling use and operational discipline as important. Apply it as evidence that these approaches can perform competitively in the stated range, not as a prediction for every climate or installation.
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What should a procurement request include?
Ask each designer or supplier for a consistent set of information so you can compare operating outcomes rather than unlike headline claims.
Quick Recap
- Annual measured PUE and WUE, with units, reporting period, IT load and system boundaries; identify any estimate separately.
- Site water use by source and season, plus the water uses included or excluded from the reported WUE.
- Climate assumptions and the expected conditions and hours for each free-cooling or economizer mode.
- Cooling configuration, including heat exchangers, chillers, towers, dry or adiabatic equipment, liquid loops and the loads served by each.
- IT density, equipment thermal limits and required air- or liquid-cooling interfaces.
- Redundancy, failure response, switchover behaviour and the operating envelope under normal and abnormal conditions.
- Control strategy, commissioning, monitoring, water treatment, staffing and maintenance requirements.
- Any additional boundary needed to assess water associated with electricity generation, electricity source or heat reuse.
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