Carbon Usage Effectiveness (CUE) is a metric introduced to the industry to address carbon emissions associated with data centers. Measuring operational carbon usage, CUE specifically measures the carbon emissions of a data center per unit of energy consumed – a critical measurement for today’s sustainability-focussed digital landscape. The metric has been introduced to the industry by The Green Grid, a global consortium dedicated to advancing energy efficiency in data centers. Working in conjunction with the ISO standard ISO/IEC 30134-8, CUE is widely viewed with importance when it comes to defining the carbon usage of a data center.

What does CUE measure?

Taking into consideration the vast range of IT equipment housed in a data center, such as servers, storage devices and network equipment, CUE takes the source energy and the quantity consumed to calculate the total CO₂ emissions; this represents the amount of greenhouse gas (GHG) emissions produced per unit of IT energy consumed.

CUE can be used in tandem with other metrics to quickly assess the sustainability and efficiency of the operations of a data center. It is however important to note it does not cover the full life cycle cost – it is a source measure of operational activity.

How is CUE measured?

The formula below is used to calculate the carbon emissions associated with the IT load of a data center, when the data center takes their entire power source from the energy grid and does not generate local CO₂.

CUE = carbon emissions (kgC0_2e) caused by the total data center energy / IT equipment energy (kWh)

The ‘total data center energy’ figure referenced above is the numerator of the power usage effectiveness (PUE) calculation – the numerator here represents the carbon emissions caused by the use of the energy in the PUE metric.

CUE is measured in kilograms of carbon dioxide equivalent, per kilowatt hour.

An alternative equation for calculating CUE is CEF x PUE, where PUE is the annual PUE figure for the data center, and CEF is the carbon emission factor of the site, based on the government’s published data for the region of operation for that year.

For any sites that generate electricity or CO₂ locally, the calculation is the same but the source data for CO₂ needs to factor in the percentage of grid sourced energy and actual emission data from local sources.

What is the ideal measurement?

A CUE of 0.0, indicating no carbon use is related to the operation of the data center, represents the ideal. However, this doesn’t represent the IT load of a data center or account for the construction process or the full life-cycle of both the data center and the IT equipment purchased and housed within. There is no upper boundary.

Why CUE is important

Designed to be used in conjunction with other data center metrics such as power usage effectiveness CUE is an effective tool for measuring infrastructure energy efficiency, and more specifically the impact that energy could have on the planet. It contributes another dimension that aids in maximizing operational efficiency and reducing negative impacts on resources.

Measuring efficiency metrics such as CUE allows data center operators to:

• Provide a standardized way to measure and report on carbon emissions
• Identify ways to improve sustainability
• Make like-for-like comparisons with other similar data centers
• Assess opportunities for the integration of renewable energy, if it’s not already in use.

Using CUE in conjunction with other efficiency metrics, organizations can implement initiatives to make operations more sustainable and assess opportunities to further reduce carbon emissions. Utilizing the information to enhance environmental status and make operational cost savings, simple improvements can include using energy-efficient IT equipment or making changes to the cooling systems; both of these examples can lower energy usage for the overall consumption of the building, which in turn can lower both the CUE and PUE.

The practicalities of measuring CUE

C0₂ emissions are based on source energy and the quantity consumed. Yet, as simple as this sounds consideration needs to be given to the fact that different energy sources have different emission factors. This can cause CUE to vary significantly according to the energy source, for example, it stands to reason that a data center powered by green energy will naturally have a lower CUE than one powered by fossil fuels.

In an ideal world, CO₂ emission data would be taken from a real time meter to deliver an exact reading, however if this data isn’t available, calculations should be made using the data available from the energy supplier, based on the load profile used.

Other efficiency metrics to consider

CUE is a helpful metric when used in tandem with other sustainability measures. It is important to consider additional indicators to form a more complete view.

• DCIM (data center infrastructure management): Software used to monitor and measure overall data center efficiency.
• WUE (water usage effectiveness): Measures water consumed for cooling.
• ERF (energy reuse factor): Highlights the extent to which expelled heat or energy is repurposed for other uses – for example, heating nearby buildings.
• REF (renewable energy factor): Gauges how much of the energy powering the data center comes from renewable sources, a vital consideration for operators looking to lower their carbon intensity.
• PUE (Power Usage Effectiveness) is a metric that measures how efficiently a data center uses energy by comparing the total facility energy consumption to the energy used solely by its IT equipment.

As the demand for cloud computing and AI continues to grow, it is important for data center operators to both understand and implement CUE alongside other metrics. Understanding their current performance allows them to make informed decisions that prioritize sustainability and efficiency, and allows them to stay competitive in an increasingly resource-conscious industry.