The Carbon Usage Effectiveness (CUE) is a metric that indicates the amount of CO2 emissions a data center generates in relation to the energy consumption of its IT systems. It quantifies the CO₂ emissions resulting from the data center’s energy consumption relative to its IT energy consumption.
Alongside the PUE (Power Usage Effectiveness) and WUE (Water Usage Effectiveness) values, it is one of the three key metrics for assessing the efficiency and sustainability of data centers. It was developed by The Green Grid and is now internationally standardized in ISO/IEC 30134-8.
To calculate it, the CO2 emissions resulting from the data center’s total energy consumption are set in relation to the energy consumption of the IT systems:
CUE = data center CO2 emissions (kg CO2e) / IT energy consumption (kWh)
The value is stated in kilograms of CO2 equivalent per kilowatt-hour (kg CO2e/kWh). The emissions include the data center’s entire energy consumption, i.e., in addition to IT, also cooling, power supply, lighting and, where applicable, backup generators.
If the data center is supplied exclusively with electricity, the metric can also be calculated in simplified form: multiply the electricity emissions factor by the PUE value.
Example: A data center with a PUE of 1.3 sources electricity with an emissions factor of 0.4 kg CO2e/kWh. This results in a value of 0.52 kg CO2e/kWh. For every kilowatt-hour consumed by the IT, 0.52 kilograms of CO2 equivalent are produced.
Each of the three metrics highlights a different aspect of data center operations:
There are relationships between the metrics, but they can also influence each other. A low PUE value automatically improves the carbon footprint as well, because less energy is required for cooling and infrastructure. Conversely, a data center with a good PUE value can still cause high emissions if it sources electricity from fossil fuels. Measures such as adiabatic cooling can reduce energy demand and emissions, but at the same time worsen the WUE.
For a holistic assessment of a data center, all three metrics should therefore be considered together.
The ideal value is 0. This means that no CO2 emissions are generated during data center operation. There is no fixed upper limit.
How high the value is depends primarily on the source of the electricity. A data center that sources electricity from renewable energy achieves significantly lower values than one operated with electricity from coal or gas. The energy efficiency of the infrastructure, i.e., the PUE value, acts as a multiplier.
A direct comparison of CUE values from different data centers is only meaningful if they were calculated using the same method.
There are two common approaches for calculating CO2 emissions:
Location-based calculation:
This uses the average emissions factor of the regional or national electricity mix. The value reflects the emissions computationally attributed to electricity consumption, based on the average regional or national electricity mix.
Market-based calculation:
This incorporates the emissions factor of the electricity actually purchased. If a data center sources certified green electricity, for example via guarantees of origin or direct power purchase agreements, the CUE can be significantly reduced.
Both approaches are valid. However, for a transparent assessment, it should always be specified which method a CUE value is based on.
CUE is a helpful metric, but it does not reflect a data center’s entire carbon footprint. It captures only the emissions from ongoing operations.
It does not take into account, among other things:
For a comprehensive sustainability assessment, CUE should therefore be supplemented by additional metrics and analyses.
Data center operators have two key levers for improving CUE: the source of the electricity and the efficiency of the infrastructure.
These include, among other things:
In Germany, sourcing electricity from renewable energy is now also required by law for data centers: Under the Energy Efficiency Act (EnEfG), operators must cover 50% of their electricity consumption from renewable energy on a balance-sheet basis since January 2024, and 100% from January 2027.
You can find out how firstcolo approaches sustainability in data center operations on our Sustainability page.
CUE indicates how much CO2 a data center causes per kilowatt-hour consumed by its IT systems. The lower the value, the more climate-friendly the operation.
The PUE value measures how efficiently a data center uses energy. CUE measures how many CO2 emissions are associated with this energy consumption. A data center can therefore be energy-efficient and still have a high CUE if the electricity comes from fossil sources.
CUE is stated in kilograms of CO2 equivalent per kilowatt-hour of IT energy (kg CO2e/kWh).
In calculation terms, yes—if the data center’s entire energy consumption is covered without emissions. In practice, this depends on whether CUE is calculated on a location-based or market-based basis and whether, for example, backup generators are operated with fossil fuels.
