Does the data centre industry need a new performance metric?
Cundall · Malcolm Howe
Cundall consultant Malcolm Howe explains why PUE is useful for trending one site but unreliable for comparing facilities, and how ITUE and TUE fill that gap.
On this page
Overview
The article was written in 2021 by Malcolm Howe, a critical-systems consultant at Cundall. It argues that PUE is useful for tracking the trend of one site over time but unreliable for comparing facilities, especially air versus liquid cooling. PUE is total facility power divided by IT power; the ideal is 1.0, and it is best used for trend tracking at one site rather than benchmarking. The author describes ITUE, which captures rack-level losses including fans, power supply units (PSUs) and voltage regulators.
ITUE was proposed by Intel's Michael Patterson and the EE HPC Working Group. TUE is ITUE multiplied by PUE. In a conventional 7 kW air-cooled rack, up to 10% of IT power can go to server and PSU fans; that share counts as IT load in PUE, so the PUE advantage of liquid cooling can be misleading. The recommendations are to meter energy at server level and to move fan power to the facility (cooling) side of the calculation.
The article ties the argument to liquid cooling and to warm-water (ASHRAE W5) operation that can remove chillers: facility water above 45 C can reduce or eliminate chillers. The article is a consultancy blog post and not a standard definition. The official PUE definition is in ISO/IEC 30134-2, while ITUE and TUE are proposed metrics. The numeric values are limited to the examples given in the article and cannot be generalised to every facility.
Key points
- PUE = total facility power / IT power; ideal 1.0; best for trend tracking at one site, not benchmarking.
- In a conventional 7 kW air-cooled rack, up to 10% of IT power can go to server and PSU fans.
- ITUE (proposed by Intel's Michael Patterson and the EE HPC Working Group) covers fans, PSUs and voltage regulators inside the rack.
- TUE = ITUE x PUE.
- Facility water above 45 C (ASHRAE W5) can reduce or eliminate chillers.
- Server-level metering and treating fan power as facility (cooling) energy are recommended.
- PUE values of air- and liquid-cooled facilities are not directly comparable.
Using it in design
- Allow server fan power to be entered as a fraction of IT load and compute TUE alongside PUE.
- Add a note that liquid-cooling PUE is not directly comparable to air-cooling PUE.
- Add a metering point option at server level in the monitoring design.
- Model the case where warm facility water (W5) reduces or removes chillers as a separate design option.
- State the method and limits clearly in PUE reports.
Monthly newsletter · free
Keep your reading list current
Once a month: new library sources, changes to Turkish regulations and upcoming data centre events.
SoSo CAD
Compare the cost of design decisions
Evaluate equipment costs, energy expenses and total cost of ownership (TCO) using your inputs and assumptions in SoSo CAD.
Request a demo
Who should read it
Energy and cooling engineers who interpret PUE results or compare air- and liquid-cooled designs.
Related standards
- ISO/IEC 30134-2 (PUE)
Publication details
- Authors
- Malcolm Howe
- Publisher
- Cundall
- Year
- 2021
- Document language
- English
- Source type
- Expert / community
- Access
- Free to read
- Licence
- Copyright retained; citation only — the publisher’s terms apply to the original document
Citation
Malcolm Howe (2021). Does the data centre industry need a new performance metric?. Cundall. https://www.cundall.com/ideas/blog/does-the-data-centre-industry-need-a-new-performance-metric
Events on this subject
- SC26 – International Conference for High Performance Computing, Networking, Storage and Analysis
- Data Center World Middle East
- Datacenter Forum Stockholm
All content and rights belong to the respective authors and publishers. SoSo CAD provides only the citation, its own summary and a link to the source, and is not responsible for the sources' content.
Related sources
All sources on this topic
Energies (MDPI AG) 2023
Cooling Technologies for Internet Data Center in China: Principle, Energy Efficiency, and Applications
In China, define project design targets from GB 50174-2017 temperature and humidity limits (18-27 °C, dew point 5.5-15 °C, RH at most 60%) and provincial PUE caps (for example Guangdong 1.25).
- Cooling accounts for about 35-40% of total IDC energy consumption, the second largest consumer after IT equipment.
Open Compute Project Foundation 2025
System Integration of a Computing Cluster, a Heat Recovery System and an Adsorption Chiller for Cold Production (OCP EMEA Summit 2025, Future Technologies Symposium)
In heat-reuse modelling, allow an adsorption-chiller sink with a minimum drive temperature (around 65 C).
- Related standard: ISO/IEC 30134-6 (ERF).
Schneider Electric 2025
10 Data Center Sustainability Improvements Quantified by Environmental Impact
In the sustainability panel compute operational CO2e = IT MWh x PUE x grid EF, with a grid-EF catalog by country/region (default US 454 kg CO2e/MWh incl. upstream) and a renewable-PPA toggle.
- Total GHG falls from 84,100 to 11,300 t CO2e (-87%).
STULZ 2024
Keeping Emmy Cool - White Paper (STULZ Modular, University of Gottingen hybrid liquid/air cooled HPC data centre)
Model the per-rack heat split (liquid %/air %); it drives both CDU and CRAH sizing.
- 11 racks installed at 96 kW, space for 14.
Huawei Digital Power 2023
White Paper on the Top 10 Trends of Data Center Facilities
Show WUE alongside PUE in the metrics panel, with region-specific targets as policy presets.
- At 16 kW per rack, power-room area roughly equals IT space (1:1).
ASHRAE TC 9.9 2021
Emergence and Expansion of Liquid Cooling in Mainstream Data Centers
Add class H1 and the W17/W27/W32/W40/W45/W+ facility-water classes to the thermal catalog as enumerations.
- Context: A1 allowable max 32 degC, A4 max 45 degC.