WP#32 - Carbon Usage Effectiveness (CUE): A Green Grid Data Center Sustainability Metric

Overview

The Green Grid's 2010 white paper no. 32 (authors include Christian Belady and Dan Azevedo) answers a question PUE cannot: two sites with identical PUE do not impose the same environmental burden when one draws from a coal-heavy grid and the other from hydropower. CUE collapses that difference into one number. It is the CO2e emitted because of the facility's total energy use (kgCO2e) divided by the energy delivered to IT equipment (kWh), written in kgCO2e/kWh.

The most practical feature is the factorisation CUE = CEF x PUE, where CEF is the kgCO2e attached to each kWh of energy entering the site. It lets you trace where an improvement came from. Lowering PUE is an infrastructure-efficiency job; lowering CEF is a matter of grid choice, supply contract or on-site generation. Because the same CUE can be reached by two different routes, reports should show both factors separately.

The ideal CUE is zero, meaning no carbon-emitting energy is used at all. Where PUE has a floor of 1.0, CUE is bounded only by the emission factor and can reach 0 with fully carbon-free supply. The definition was later carried into ISO/IEC 30134-8, so the white paper is the historical source while the standard is the text to cite in reporting today. The full paper requires registration, so this page rests on the definition and the factorisation.

Key points

1. CUE = total CO2e from all data centre energy (kgCO2e) / IT equipment energy (kWh); unit kgCO2e/kWh.
2. Equivalent form: CUE = CEF x PUE, with CEF the kgCO2e per kWh of site energy.
3. Ideal value is CUE = 0, i.e. no carbon-emitting energy.
4. The factorisation separates the effect of grid/energy supply from the effect of infrastructure efficiency.
5. The definition is standardised in ISO/IEC 30134-8; GHG Protocol Scope 2 guidance is the related accounting framework.

Using it in design

• Compute CUE as grid emission factor (kgCO2e/kWh, per region and hour) x PUE.
• Add emissions from on-site fuel burning such as generator testing and CHP; using the grid factor alone understates CUE.
• Show CUE next to PUE and WUE in the same summary so an option that cuts carbon while raising water use is visible.
• Let users choose between location-based and market-based (PPA) emission factors, and state which one a report used.

Who should read it

Facility engineers preparing sustainability reports and decision makers comparing energy supply contracts.

Related standards

• ISO/IEC 30134-8
• GHG Protocol Scope 2 Guidance

Publication details

Authors

Christian Belady; Dan Azevedo; Michael Patterson; Jack Pouchet; Roger Tipley

Publisher

The Green Grid

Year

2010

Document number

WP 32

Document language

English

Source type

Industry body

Access

Citation only

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation