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Glossary
Data centers

What is PUE (ISO/IEC 30134-2)?

Definition

Power usage effectiveness (PUE) is the data centre energy efficiency KPI defined in ISO/IEC 30134-2: total facility energy divided by the energy used by IT equipment.

How it is calculated

PUE equals total data centre energy divided by IT equipment energy over the same period. Since IT energy is part of the total, the ratio cannot go below 1.0, and everything above it is cooling, power conversion, lighting and other support load.

ISO published the second edition, ISO/IEC 30134-2:2026, in January 2026. It replaces the 2016 edition and its 2018 amendment, adds the mPUE variant for mixed-use buildings, and clarifies unmeasured energy and on-site generation.

Metering behind the ratio

A PUE is only as good as the sub-metering under it. The Aqueduct electrical twin balances grid, solar and generator input against each feeder, so IT load, cooling and losses each get a measured term, with transformer loss estimated. Its sub-metering coverage figure shows how much of the total is metered before the ratio goes into a report.

Questions

PUE (ISO/IEC 30134-2): common questions

PUE is the most widely used energy efficiency metric for data centres, standardized in ISO/IEC 30134-2. It shows how much energy goes to cooling, power distribution and other overhead compared with the IT load itself, so operators can track overhead over time. Aqueduct's view is on the data centers page.

For energy overhead, yes, but PUE ignores water. Switching to evaporative cooling can lower PUE while raising water use, which is why operators read it next to WUE and look at both before changing cooling strategy.

Not under the standard. ISO/IEC 30134-2 sets measurement categories and reporting rules so figures compare fairly, and a spot reading misses seasonal swings in cooling load. Annual figures from continuous metering are the reliable basis for reporting.

PUE is an operating metric measured over time. ASHRAE 90.4 is a design standard that limits mechanical and electrical overhead before a facility runs. A data centre can meet 90.4 at design and still drift in operation, which continuous metering of each feeder reveals.

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