Monitoring
PUE, power usage effectiveness, is the total energy a data centre uses divided by the energy that reaches its IT equipment. A PUE of 1.0 would mean every kilowatt-hour goes to IT; anything above that is the overhead of cooling, power conversion, lighting and losses. The UPS sits in the middle of that overhead and is usually where measurement starts.
PUE is the ratio of the total energy a data centre facility uses to the energy used by its IT equipment. The Green Grid, which defined the metric, puts it in one line: PUE for a dedicated building is the total facility energy divided by the IT equipment energy. Total facility energy is everything measured at the utility meter for the data centre: the IT equipment itself, plus the power delivery chain (UPS systems, switchgear, generators, PDUs, batteries and distribution losses), plus cooling, lighting and the rest of the support load.
The result reads as a multiplier. The Green Grid's own illustration: a PUE of 3.0 means the data centre uses three times as much energy as its IT equipment alone. A value approaching 1.0 would mean almost all energy reaches IT. Its inverse, DCiE (data centre infrastructure efficiency), divides IT energy by total energy; the two say the same thing in different forms.
PUE is calculated from energy over a period, in kilowatt-hours or megawatt-hours, not from a single power reading. The Green Grid explains why: energy is power multiplied by the time it is applied, so a figure built from energy reflects the whole period rather than one moment. The example below is its own, from White Paper 49: a containerised data centre with a UPS that is 95% efficient, a transformer that is approximately 96% efficient, and a chiller outside the container.
| Where the energy goes (one year) | Energy (MWh) | Counted as |
|---|---|---|
| Transformer losses | 25 | Facility overhead |
| Chiller | 75 | Facility overhead |
| UPS losses | 25 | Facility overhead |
| IT equipment | 475 | IT equipment energy |
| Total delivered to the transformer | 600 | Total facility energy |
PUE is therefore 600 divided by 475, or 1.26. The UPS on its own wastes as much energy in this example as the transformer does, which is why UPS efficiency and UPS load level show up in PUE. Notice what the same figures give if you draw the boundary around the container only: (25 + 475) / 475 = 1.05. The Green Grid calls that a partial PUE and warns that a quoted figure of 1.05 or similar is often the partial PUE of one zone, not the PUE of the whole facility.
At one of three points, and the point sets the level of the result. The Green Grid defines:
Total facility energy is measured at the utility input at every level. Moving from Level 1 to Level 2 takes the losses of the PDUs and their transformers out of the IT figure and puts them where they belong, in the overhead. ASHRAE's paper on power equipment notes that many PDUs already have built-in power monitoring whose data can be used to calculate PUE, so Level 2 is often a matter of collecting what the equipment already reads.
It does not tell you how much useful work the IT equipment does, and it can move the wrong way after a good decision. The Green Grid gives the classic case: virtualising servers can reduce the total IT load and still raise PUE, because the cooling and power overhead does not fall in proportion. The facility uses less energy overall, yet the ratio looks worse.
PUE also depends on when and how it is measured. The Green Grid recommends taking Level 1 and Level 2 readings at about the same time of day, when the load is as consistent as possible with earlier readings, and keeping the day of the week constant for week-to-week comparisons. Comparing your PUE with another site's makes sense only when both use the same level, the same period and the same boundary; a container's partial PUE and a building's PUE are not the same number.
Two energy measurements at minimum: total facility energy at the utility input, and IT equipment energy at the chosen level. The Green Grid asks for a kilowatt-hour meter that reports true energy, by measuring voltage, current and power factor together over time, rather than a current reading multiplied by an assumed voltage. In many sites the UPS display or its network card is the first IT meter available; Lawrence Berkeley National Laboratory's guide on energy information systems notes that points not metered automatically are often recorded by hand from equipment displays such as the UPS display.
Collecting those readings automatically is where monitoring software comes in. The difference between a DCIM platform and a building management system for this job is covered in DCIM vs BMS; the sensors that sit around the same racks are in the server room monitoring guide.
By wasting less energy between its input and its output. In the worked example the UPS losses equal the transformer losses, so a more efficient UPS, or one running at a better point on its efficiency curve, cuts overhead directly. Efficiency depends on the UPS design, its operating mode and how heavily it is loaded; the data centre power architecture guide explains where the UPS sits in the power path. When an older UPS is due for replacement, efficiency at your real load is worth putting into the comparison; maintenance for data centre UPS systems covers the maintenance side of that decision.
Lower is better, and 1.0 is the floor. There is no single target: the figure depends on climate, cooling design, redundancy and load. Compare your own PUE over time, at the same measurement level, before comparing it with other sites.
In energy. The Green Grid's examples use energy over a period, such as a year in megawatt-hours, because power is only a snapshot of one moment.
Overhead. UPS systems, switchgear, PDUs, batteries and distribution losses are all part of total facility energy. At Level 1 the IT load is read at the UPS output, after the UPS losses.
Because the IT load fell faster than the cooling and power overhead. The Green Grid gives this as an example: total energy can drop while the ratio rises.
PUE calculated for one zone, such as a container or a room, rather than the whole facility. In the Green Grid's example the container's partial PUE is 1.05 while the facility's PUE is 1.26.
Sources, checked 2026-10-04:
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