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Data centre power and cooling depend on the IT load, supporting systems, climate, water supply and resilience requirements—not on one universal design or efficiency score. Building owners should measure energy at a clearly defined boundary, track water as well as electricity, and use operating metrics such as PUE to follow their own facility over time rather than to rank unlike buildings.
This guide explains what uses energy, how to interpret the main metrics, what to inspect in an existing site, and how to assess cooling options before a build or retrofit. Requirements and equipment limits vary by site; confirm the applicable code, IT guidance and operating constraints with qualified professionals.
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What equipment uses energy in a data centre?
Servers, storage and networking equipment use electricity to process, store and move data. The facility also uses energy to remove the heat those devices produce and to support reliable operation. These supporting loads can include chillers, cooling towers, pumps, fans and air-handling equipment, as well as UPS and other power equipment, lighting and ancillary building systems.
The balance varies. A data centre inside an office or research building may share plant and have a different load mix from a large standalone facility. IT demand, cooling design, climate, utilisation and redundancy all affect the total. That is why a generic percentage split between IT and infrastructure is not a reliable baseline for an individual building.
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For an owner, the first practical distinction is between IT energy and total facility energy. The first is the electricity drawn by the IT equipment within the agreed measurement boundary; the second includes that energy plus the power needed by the facility’s supporting infrastructure. If the two readings come from different boundaries or time periods, ratios calculated from them can mislead.
What is PUE, and what does it tell an owner?
Power Usage Effectiveness (PUE) is total data centre facility energy divided by IT equipment energy. It is useful for tracking infrastructure overhead at the same facility over time, but it does not say how efficiently the IT equipment performs useful work, how much water the facility uses, or what its carbon impact is.
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Do not treat that lower bound as a practical target or use a single PUE number as a complete verdict on a building. ASHRAE says PUE was developed for an operation to monitor its own energy efficiency and notes that climate zone and redundancy affect the result. An older or hotter-climate site, or one built with a different resilience design, may not be comparable with another site even if both report the same metric consistently. PUE also excludes IT work efficiency, water and carbon, so it cannot establish which facility is more sustainable overall.
For context, Uptime Institute reported a weighted average annual PUE of 1.54 among respondents to its 2025 survey. That is a survey result, not a target, guarantee or representative value for every facility. The same survey stresses the diversity of facilities by size, age and region. Separately, the Department of Energy’s 2024 guide reports a 1.55 annual average for large data centres in 2022, citing Uptime Institute’s 2022 data; the figures refer to different survey contexts and years and should not be blended into a single trend.
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Which metrics should be tracked alongside PUE?
Pair PUE with measures that cover water, carbon and energy reuse, and define each metric’s boundary before comparing periods or sites. The Department of Energy’s guide includes WUE, ERE and CUE among its data-centre performance metrics.
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- WUE (Water Usage Effectiveness): Site-based WUE is annual site water use divided by IT energy, expressed in litres per kilowatt-hour. State the water boundary and what counts as site water; a WUE figure without those definitions is difficult to interpret.
- ERE (Energy Reuse Effectiveness): This accounts for facility energy after energy reuse. It can add context where useful heat is exported or reused, but the accounting needs to make the reuse arrangement clear.
- CUE (Carbon Usage Effectiveness): This is a carbon-related metric. Interpret it alongside clearly stated energy and emissions boundaries rather than as a standalone label.
Water and electricity can trade off. For example, reducing on-site evaporative water use can change energy needs or the cooling approach. Local water availability and stress therefore belong in the decision, not just the water bill. Cooling-tower consumption also relates to IT and other heat loads and to how efficiently that heat is removed. Owners should review water and energy together rather than declaring a design better based on one figure.
Where should IT energy be measured?
For the U.S. Environmental Protection Agency’s ENERGY STAR data-centre rating methodology, IT energy is generally measured at the UPS output. A general room submeter is not automatically an IT-energy meter: it may include lighting, space conditioning or other non-IT loads.
The EPA’s ENERGY STAR rating FAQ describes exceptions when a site has no UPS or when non-IT loads on the UPS cannot be separated, including use of PDU input in those circumstances. Check the rating instructions and the actual electrical arrangement before selecting the boundary. This is methodology for a particular rating; it should not be assumed to define every owner’s accounting boundary or local reporting obligation.
Document which meters feed each reported number, what equipment sits downstream, and whether the readings cover the same period. Keep that method consistent when assessing change. If a meter, UPS configuration or load mix changes, record the change so a new ratio is not mistaken for an efficiency gain or loss.
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How can owners reduce cooling energy and water use?
Start by checking airflow, temperature and humidity control before proposing a major equipment replacement. Poor separation of hot and cold air, unnecessary cooling demand or overly tight environmental control can waste energy and, where heat rejection uses water, contribute to additional water use.
- Establish the operating baseline. Review utility bills and submeter data, UPS/PDU readings, cooling and power redundancy, temperature and humidity trends, alarms, maintenance history and cooling-water use where applicable. Confirm the meter boundaries before comparing readings.
- Look for airflow problems. Trace how supply air reaches IT equipment and where hot exhaust returns. Check for bypass air, recirculation or mixing between hot and cold zones, and whether the physical layout and operating controls support separation. Fixes depend on the room and equipment; do not assume that a particular containment arrangement suits every site.
- Review setpoints against equipment requirements. The Department of Energy’s Federal Energy Management Program warns that data-centre temperatures are often controlled well below recommended setpoints while humidity is sometimes kept in an unnecessarily narrow range. Lower temperatures and tighter humidity control can increase chiller demand and cooling-tower water use.
- Assess the whole cooling system. Review economizer opportunities, control sequences, fan and pump operation, and how the heat is rejected. An air-side or water-side economizer may reduce compressor operation where the climate and design permit, but actual savings depend on local conditions and system operation.
- Validate changes before relying on them. Adjustments must remain within IT equipment guidance and protect reliability and operational availability. Use operating data and a qualified commissioning or energy specialist when a change affects a complex plant or a critical load.
A temperature and humidity data logger can help observe room conditions, but it is an observation aid—not a replacement for calibrated facility sensors, building controls or a commissioning professional. DOE’s cooling-water guidance also supports looking at energy and water together rather than treating setpoint changes as a simple electricity-only intervention.
Which cooling approach fits a new build or retrofit?
There is no universal best cooling system. Compare options against the site’s climate and water constraints, heat load and rack density, resilience requirements, available space, existing distribution systems, retrofit disruption, capital and operating cost, and potential to reuse heat.
| Approach | Potential value | Questions and trade-offs |
|---|---|---|
| Air cooling | Can suit a range of established facilities and may be integrated with air-side economizer operation where climate and design allow. | Can airflow reach and remove heat from the actual equipment load? Are hot and cold air separated effectively? Is there room to modify the air path? |
| Evaporative heat rejection | Can support heat rejection using water, with the design and local conditions determining overall performance. | How much water will the system consume at the expected heat load? Is the supply dependable and acceptable in a water-stressed location? |
| Dry cooling | Can reduce on-site water demand compared with a water-consuming heat-rejection approach. | What are the energy and climate implications at this site, and can the equipment meet the required load and resilience needs? |
| Economizers | Air-side or water-side economizer operation can reduce compressor use when ambient conditions and system design allow. | How many useful operating hours can the site conditions support? What controls and maintenance are needed to use the opportunity reliably? |
| Liquid cooling | Moves heat from high-density IT equipment; it is especially relevant to high-density AI facilities. | Does the design integrate the IT equipment, technology cooling system and facility heat rejection? How will maintenance, redundancy, water use and possible heat reuse be handled? |
Liquid cooling is not a standalone answer to facility heat rejection: heat still has to be removed or reused at the building level. ASHRAE’s AI Data Center Energy Performance Framework discusses direct-to-chip cooling, rear-door heat exchangers, technology cooling systems, dry coolers, low-water approaches, heat reuse and monitoring. Its AI-specific density examples should not be generalized to a conventional enterprise room.
For a retrofit, compatibility matters as much as theoretical performance. Owners need to consider whether existing distribution, plant, controls and available space can support the change, how work affects live operations, and whether the design can be maintained and serviced. No universal cost or payback period applies; those depend on the site, design and operating assumptions.
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What should owners inspect at an existing data centre?
A useful review connects meter data to the physical plant and the service the facility must deliver. Before promising savings, establish a measured operating baseline and identify the conditions under which it was collected.
- Energy boundaries: Map utility and submeter readings, UPS output and PDU input. Identify non-IT loads included in each measurement and note any unmetered or shared building services.
- Cooling operation: Examine temperature and humidity trends, airflow paths, control sequences, fan and pump operation, economizer use, alarms and maintenance records.
- Water: Where cooling water is used, identify the relevant meters and uses, and review consumption alongside heat load and energy data.
- Resilience: Document cooling and power redundancy, how maintenance is carried out, and which failures the design is intended to withstand.
- Future demand: Confirm current IT load, expected growth and rack-density needs before deciding whether existing capacity and cooling distribution remain suitable.
A temperature and humidity logger can supplement observations, but it should not replace calibrated sensors or controls. A specialist energy assessment or commissioning review can help when systems are complex, readings conflict or a significant retrofit is under consideration. A site-specific assessment is more dependable than applying a generic savings promise to an unmeasured facility.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should be confirmed before a build or major retrofit?
Before committing to a design, confirm the load, site constraints, performance goals and approval path together. The Department of Energy recommends total-cost-of-ownership analysis and multiple efficiency and sustainability metrics; its guidance does not support a universal retrofit cost or payback period.
- Define current IT load, expected growth, rack density and service availability needs.
- Confirm utility capacity, metering boundaries, cooling architecture, available space and heat-rejection options.
- Assess local climate, water supply and water-stress conditions, including the energy-water implications of alternatives.
- Set redundancy and maintainability expectations, and test how they affect both design and performance metrics.
- Consider controls, monitoring, heat-reuse opportunities and the operating expertise needed to maintain the selected system.
- Compare lifecycle costs and several metrics, not PUE alone, using stated assumptions about energy, water, emissions and operating conditions.
- Verify the locally adopted code path and required approvals with the authority having jurisdiction and qualified design professionals.
ASHRAE’s handbook describes Standard 90.4 as a data-centre efficiency standard and notes that using it as an alternative compliance path depends on recognition by the relevant code jurisdiction. The edition adopted locally and the compliance route can vary. Treat this as a prompt to verify local requirements—not as jurisdiction-specific legal advice.
How should owners assess resilience claims from a colocation provider?
Ask how power and cooling redundancy are configured, which failures the design is meant to withstand, how maintenance is handled and exactly what the service-level agreement (SLA) promises. Redundancy can differ materially between colocation centres, so assess the actual site and contract before signing rather than inferring resilience from a general label.
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Clarify whether a commitment covers availability, maintenance or a specific response to an outage, and which exclusions or service boundaries apply. Compare the contract language with the physical design and the service your building requires. An SLA is a contractual commitment; it does not by itself describe every equipment path or operational failure mode.
Frequently Asked Questions
How often should a building owner review data centre energy readings?
Use a full annual period when calculating PUE so seasonal cooling conditions and changing IT loads are represented. More frequent operational reviews can help identify unusual changes, but compare like periods and keep the measurement boundary consistent.
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Not automatically: a room or landlord meter may include lighting, space conditioning or other non-IT loads. Follow the rating method’s boundary rules and document any exception, such as the absence of a UPS or inseparable non-IT loads.
Does a lower PUE mean a data centre is more sustainable?
Not on its own. PUE describes facility energy relative to IT energy, but leaves out IT work efficiency, water use and carbon; compare those dimensions with clearly defined boundaries as well.
What questions should I ask before changing a data centre temperature setpoint?
Ask which equipment requirements and operating limits apply, how temperature and humidity are measured across the room, and how the change will be checked for reliability. Include cooling-water effects in the review if the site uses water-based heat rejection.
