Power Usage Effectiveness (PUE) is the most quoted efficiency figure in the data centre industry, and one of the most misused. Used properly, it is a practical tool for deciding how large a cooling upgrade needs to be and what it is likely to save. This article sets out a method. | What PUE measures. PUE is defined in ISO/IEC 30134-2 as total data centre energy divided by the energy delivered to the IT equipment. A PUE of 1.0 would mean every kilowatt hour reaches the IT load; everything above 1.0 is overhead, mainly cooling, power conversion losses and lighting. | Two points matter before any design decision: | Use energy, not power. PUE should be calculated from energy over a period, ideally a full year, so that seasonal cooling demand is included. A single instantaneous reading on a cool day will flatter the site. | Measure at the right points. IT energy should be metered at the IT equipment input, typically at the PDU or rack level, not at the UPS input, otherwise UPS losses are counted as IT load. | A worked example. The figures below are illustrative, to show the method. A data hall consumes 8,760 MWh of IT energy in a year, an average IT load of 1,000 kW. Total facility energy is 13,140 MWh, so the annual PUE is 1.5 and the overhead is 4,380 MWh. | The next step is to split that overhead. Sub-metering, or a calibrated estimate where meters are missing, might show: | Cooling plant and air movement: 3,300 MWh | UPS and distribution losses: 800 MWh | Lighting and ancillary loads: 280 MWh | Cooling is now visible as the dominant overhead. A useful sub-metric is the partial PUE for cooling, the cooling energy plus IT energy divided by IT energy, here about 1.38. | Sizing the upgrade. Set a target and work backwards. If the cooling partial PUE can be brought from 1.38 to 1.20 through higher supply temperatures, containment and free cooling hours, cooling energy falls from 3,300 MWh to about 1,750 MWh a year. That saving of roughly 1,550 MWh is the figure to test against the capital cost of the works, and it tells the design team how much plant capacity and how many free cooling hours the new scheme must deliver. | Check the result against the IT roadmap. If rack densities are rising, the upgrade has to be sized for the future load, not the current one, and liquid cooling may change the split between air and water side energy entirely. | Reporting obligations. In the EU, operators of data centres with an IT power demand of at least 500 kW must report energy performance indicators to a European database under Delegated Regulation (EU) 2024/1364, with the first report due on 15 September 2024 and annual reports from 15 May 2025. A site that already meters PUE correctly is well placed to meet that obligation. | Next steps. Our free engineering tools include calculators for early stage checks. For an independent review of a data hall's cooling strategy, contact our team.