PUE, WUE and CUE, explained
The three facility ratios most often quoted for data centers, what each one measures, and what none of them can tell you. Sources checked 5 November 2026.
Three ratios, three questions
| Ratio | Question | Formula (in words) | Better |
|---|---|---|---|
| PUE, power usage effectiveness | How much facility energy does each unit of IT energy need? | Total facility energy ÷ IT equipment energy | Lower; never below 1 |
| WUE, water usage effectiveness | How much site water is used per unit of IT energy? | Site water use ÷ IT equipment energy | Lower |
| CUE, carbon usage effectiveness | How much carbon is emitted per unit of IT energy? | Emissions from facility energy ÷ IT equipment energy | Lower |
The PUE standard
ISO/IEC 30134-2:2026, published on 15 January 2026, covers PUE measurement categories, measurement, calculation, reporting and interpretation (Standards Council of Canada record for ISO/IEC 30134-2:2026; ISO/IEC 30134-2:2026). PUE is calculated from energy over a period, not from a single power reading. Other parts of the ISO/IEC 30134 series define further indicators; check the ISO catalog for current editions.
Why one number can mislead
- Boundary: a PUE that leaves out part of the facility is a partial PUE and must say so.
- Period: a cool month can look far better than a full year; annual claims need twelve months of energy.
- Metering point: IT energy measured at the rack and at the server inlet differ.
- Nothing about work done: idle servers improve PUE. MLPerf Power rejects rated power and PUE as substitutes for measured power (Tschand et al., MLPerf Power).
How the Joule Ledger maps to them
JL-26 is a proprietary method that maps to these definitions. Its Overhead Ratio at a data center is E(M4) ÷ E(M3), which equals PUE when the boundaries match the standard. WUE and CUE share the same denominator, IT energy at M3, so they inherit the same meter-point discipline. JL-26 pairs them with Decision Yield, so a facility cannot look efficient while doing no useful work.
Where the EU uses them
The EU rating scheme grades energy with PUE and water with WUE, on separate A-to-G scales. See the EU rating scheme. Reporting under Delegated Regulation (EU) 2024/1364 applies to data centers of at least 500 kW.
Questions
Can PUE be below 1?
No. Total facility energy always includes the IT energy, so the ratio is at least 1. A figure below 1 means something has been left out or credited, such as reused heat, which is reported separately.
Does a low PUE mean a facility is efficient?
It means little energy goes to overhead. It says nothing about whether the IT energy did useful work.
Sources
Keep reading
Take the next step.
What PUE, WUE and CUE measure, the PUE formula in ISO/IEC 30134-2:2026, why one number can mislead, and how the Joule Ledger maps to each.