"PUE 1.09" — big type on the sustainability page. The CSR lead smiles during the internal presentation. Yet that figure says nothing about where the electricity comes from, nothing about reusing waste heat, nothing about the impact of manufacturing servers — and nothing about your overloaded WordPress CPU draw running twenty-four hours a day.
PUE is a useful tool, but it is regularly turned into a final verdict on a host's environmental credentials. Understanding what it measures — and especially what it excludes — keeps you from validating an infrastructure choice on a single reassuring metric.
Definition — and what the ratio actually includes
PUE = total datacenter energy ÷ IT energy.
A PUE of 1.5 means that for 1 kWh actually used by servers, the site consumes 1.5 kWh once cooling, lighting, and losses are counted. The best documented sites approach 1.1 — Infomaniak communicates around 1.085 on its Swiss infrastructure.
PUE measures building and auxiliary efficiency. It does not cover:
- the electrical carbon mix feeding the site;
- upstream emissions from manufacturing and transporting hardware;
- the useful load of your VMs or containers;
- reuse of residual heat (ERE or ERF ratios).
| Displayed PUE | What it proves | What it does not prove |
|---|---|---|
| 1.05–1.15 | Controlled cooling | Carbon neutrality |
| "Better than average" | Building comparison | Overall project footprint |
| Annual average | Site trend | Undetailed summer peaks |
A good PUE is a necessary condition, never a sufficient one, for credible green hosting.
Frequent communication bias
"Marketing" PUE versus measured PUE. Without a recognised protocol (ISO/IEC 30134), the figure may be a design target, not continuous audited measurement. Ask for the method, measurement period, and who validates the numbers.
Site PUE versus hall PUE. A new ultra-efficient building can hide aging IT elsewhere in the group, or halls excluded from the communicated perimeter.
PUE without energy mix. Electrical efficiency and decarbonisation are not synonyms. Guarantees of origin bought beside a low PUE do not solve everything if actual generation remains carbon-intensive.
Silence on heat reuse. Advanced players like Infomaniak remind us that PUE alone is insufficient for climate issues — heat reinjected into a district network matters as much as optimised cooling.
ERE, ERF, and what PUE cannot see
Energy Reuse Effectiveness (ERE) and Energy Reuse Factor (ERF) measure how much waste heat from the datacenter is valorised — district heating, greenhouses, neighbouring buildings. An excellent PUE with low ERE means electricity is converted efficiently into cooling, but heat still goes up the chimney.
For a credible CSR policy, cross three indicator families:
| Indicator | Question asked | Limit |
|---|---|---|
| PUE | Does the building waste energy? | Ignores carbon and your apps |
| Renewable mix | Where does electricity come from? | Says nothing about internal efficiency |
| ERE / ERF | Is heat reused? | Few hosts publish it |
Our investigation Infomaniak ecology: proof or promises shows how to read these documents beyond the hero figure.
The peak: PUE optimises the building, not your responsibility
The most frequent risk is not the absence of a figure. It is the illusion of coverage: you believe you chose a sustainable host because a page shows 1.09, while the power mix, scope 3 hardware, and application overconsumption remain off the books.
Decide and move forward without blind spots
Before your next supplier review or CSR tender, build a dossier in four parts. Ask for PUE with the measurement method (ISO/IEC 30134 if possible), verifiable renewable share, a dated scopes 1–3 carbon report, and ERE or ERF ratios if the site claims heat reuse.
Then confront those elements with your own load: right-sizing instances, shutting down test environments overnight, log retention policy. A green host with oversized VMs still leaves a heavy footprint. Compare providers via the directory and the Infomaniak ecology investigation so you do not stop at the first hero number.
Frequently asked questions
What exactly is PUE?
Power Usage Effectiveness is the ratio between total energy consumed by a site (IT, cooling, auxiliaries) and energy used by IT equipment. The closer to 1, the more efficiently the building converts electricity into useful load. The global average sits around 1.5 to 1.6.
Does a low PUE guarantee green hosting?
No. PUE measures datacenter energy efficiency, not carbon mix, heat reuse, or your application consumption. A PUE of 1.1 fed mostly by coal remains polluting in climate terms.
Why do hosts put PUE front and centre?
It is a simple, comparable indicator, often flattering for recent facilities. It lets providers talk about efficiency without addressing scope 3 hardware or client overconsumption — topics harder to summarise.
Which indicators should complement PUE?
Ask for verifiable renewable share, ERE or ERF ratios, a dated scopes 1–3 carbon report, and hardware renewal policy. Cross-check these with your real workload footprint.
PUE in the hero? Ask for the electrical mix on the same slide — or suspect cherry-picking.