PUE in an edge micro data centre: why that number in your specification does not hold

The specification for the edge site you are about to put out to tender has a line on energy efficiency, and that line asks for a PUE. You will verify that number at acceptance. The Recommendation that deals with exactly those sites — micro data centres, not the large halls — states that PUE must not be used there as it is. The acceptance test you agreed measures the wrong thing, and you will find out when the supplier hands you a low value obtained by leaving equipment out of the installation.

What counts as a micro data centre

Recommendation ITU-T L.1307, Energy efficiency in micro data centres for edge computing, edition 03/2024, was approved on 8 March 2024, is listed as In force in the catalogue and is freely downloadable. It defines the object like this: “A solution designed to provide processing, storage and networking capabilities in a more compact and modular form”.

The definition carries no threshold: no kilowatts, no floor area, no number of cabinets. That is the first line missing from most specifications: if you do not write it yourself, micro becomes whatever suits the bid. Clause 7 lists where these installations actually end up: an ordinary office, an equipment room, a base station site, an exchange; inside the building or outdoors. Places where a data hall was never designed, and where the protections that are obvious elsewhere have to be asked for one by one.

Why PUE measures so little there

Clause 9 is blunt: “micro data centres may not have a backup power supply or cooling system to a similar level to the typical data centre, so this indicator should not be used as it is”.

The reason is arithmetic. PUE is the ratio between the total energy of the site and the energy of the IT equipment: if the site has no UPS, no standby generation and no dedicated cooling, the numerator is missing precisely the items PUE exists to weigh. A low PUE, in an edge cabinet, may only mean that the infrastructure is not there at all, and that the site is exposed rather than efficient. And the usual question remains, where the meters go.

The indicator L.1307 puts in its place

The alternative is a ratio between two quantities that are already measured:

EEI(MDC) = ITEUsv / PUE

ITEUsv is the average CPU utilisation of the N servers running at that instant — in formula (2), the sum of Ui(t) divided by N — and it is taken from ISO/IEC 30134-5. The higher EEI, the better the efficiency: consolidating the workload onto a few well-utilised servers weighs as much as cutting infrastructure consumption.

There are three measurement prescriptions, and they belong in the acceptance record. ITEUsv is sampled at defined intervals over a given measurement period, “for example, a year”. At each interval you take the peak, not the average: “The maximum value of ITEUsv(t) should be used at every measurement time interval”. And it covers servers only: “it does not apply to storage systems or network equipment in a data centre”.

Then comes the gap: L.1307 gives no target value for EEI. The words target and threshold never appear in the text. The indicator ranks, it does not pass or fail anyone. The threshold that makes the acceptance test enforceable is one you write yourself.

One single obligation in the whole document

Worth knowing before you refer to the Recommendation with a single dry line. In the body of L.1307 the word shall — the one that, under ITU-T conventions, expresses a requirement — appears exactly once, in Table 1, on sizing the power supply for the maximum power drawn by servers, storage, network, cooling and security equipment. Everything else is twenty-two instances of should and six of recommended. If you want them enforceable at acceptance, you are the one who has to transcribe them as obligations.

Four of them deserve to go into the specification as they stand.

Separate electrical distribution: “It is recommended to separate electrical power distribution between the data centre and other service present in the site”. In an office that is the difference between a measurable site and a meter that also weighs the coffee machine.
Autonomy: “It is desirable to provide the battery capacity of the UPS to provide a backup time of 5–10 minutes to protect from power outage”. Five to ten minutes is not an autonomy figure: it is the time to switch to another source, and the specification has to say which one, and with what capacity test.
Expansion: “Before physical expansion, computing offloading to a nearby micro data centre or a central cloud data centre is recommended”. Before adding cabinets, the workload moves elsewhere. That is a contractual clause: it changes who pays for growth.
Noise and vibration in an office: the Recommendation asks that they should not hinder the people working in the same room, but gives no limit. The decibel figure is yours to write.

The paid standards, and the edition that has changed

L.1307 cites two ISO/IEC standards as normative references: ISO/IEC 30134-2:2016 for PUE and ISO/IEC 30134-5:2017 for ITEUsv. They are paid documents, we have not read them, and of them we report only designation, title, edition, date and catalogue status.

The catalogue, though, says something that matters. ISO/IEC 30134-2:2026, edition 2.0, is recorded as published on 16 January 2026: it is a second edition, and L.1307 refers to the first. ISO/IEC 30134-5:2017 carries an Amendment 1 published on 11 March 2025, later than the Recommendation. Clause 2 gives the warning itself: “users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition”.

Translated into a specification: a reference to PUE according to ISO/IEC 30134-2, with no edition and no year, leaves the choice to the supplier, and at acceptance you receive a number calculated with a method that is not the one you had in mind. In clause 9 the cross-reference is moreover printed with one digit too many, 303134-5: a reference that does not exist binds nobody.

What we do not know

Where the micro ends. The definition fixes no power, no floor area and no number of cabinets: the boundary stays contractual.
What a good EEI is worth. With no target value, the indicator compares two sites but promotes neither.
Under which measurement category. L.1307 uses PUE in the denominator but does not say which measurement category applies in a site with no dedicated infrastructure.
The content of the paid standards. Of ISO/IEC 30134-2 and 30134-5, new editions included, we know only the catalogue data: if the method has changed, we cannot say so.
Noise and vibration. No numerical limit in the Recommendation.

The point

An edge site is accepted or disputed on three lines of specification: which indicator, with what sampling interval and measurement period, against what value. In the compliance specifications we deliver those lines are there, with the edition and year of the standards referred to and the recommendations you want enforceable transcribed as obligations — from separate distribution to the minutes of battery backup — and the verification is done at acceptance, with the record and the documentation handed over together with the work. On the other axis, the data that Table 2 of L.1307 asks you to collect — location, age and specification of servers, storage, network and racks, utilisation, consumption, temperatures — today sit in an export, a spreadsheet and the installer’s as-built. With us they converge into a single map of the site and the network, where an AI flags the server idling at minimum load that is holding ITEUsv down, and the drift that by year end makes you miss the agreed value, and one of our own teams goes out to fix it. Where an AI is needed on that data, it runs inside the client’s perimeter, on-premise or in a dedicated cloud with data centres in Italy, together with CSIDIA, the other company in the group.

Are you writing the specification for an edge site, or accepting one already built, and do you want an indicator that holds up under challenge? Talk to an engineer: the site survey is free of charge, and those three lines are decided before the tender.