Rack & DCIM
PUE Reference
PUE and DCiE formulas, Green Grid efficiency tiers, typical values by facility type, where overhead power goes and a PUE-to-DCiE conversion table. Use alongside the PUE Calculator.
Last updated: June 2026
Terms
- PUEPower Usage Effectiveness
- How much total electricity a facility uses for every unit that reaches the actual computers. 1.5 means 50% extra goes to cooling and power losses. Lower is better.
- DCiEData Center Infrastructure Efficiency
- The flip side of PUE, shown as a percentage instead of a ratio. 100% would mean every watt reaches the computers; real facilities are usually 50–90%. Higher is better.
- ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers
- The US industry body that publishes the standard temperature and humidity ranges data-center equipment is designed to handle. When a chart says "ASHRAE recommended range", it means the safe zone most server makers design for.
- CRACComputer Room Air Conditioner
- The specialized air-conditioning unit that cools a server room.
- CRAHComputer Room Air Handler
- Similar job to a CRAC unit — it cools a server room — but instead of cooling the air itself, it uses chilled water piped in from elsewhere in the building.
- IECInternational Electrotechnical Commission
- The international body that writes electrical safety standards used across most of the world outside North America. When a chart cites an IEC number, it's pointing to the official rulebook behind a calculation.
- PDUPower Distribution Unit
- Essentially a heavy-duty power strip built into a server rack, splitting one incoming power feed into multiple outlets for the equipment inside.
The PUE Formula
PUE = Total Facility Power ÷ IT Equipment Power
Total Facility Power = IT + cooling + electrical losses (UPS/PDU/transformer) + lighting & ancillary
DCiE = 1 ÷ PUE × 100% — the share of facility power that reaches IT equipment
Overhead = Total − IT — everything that is not useful IT work
The ideal PUE is 1.0 (all power reaches IT). PUE is most meaningful as an annualised average from 12 months of metered energy, per The Green Grid and ISO/IEC 30134-2.
Green Grid Efficiency Tiers
Common industry interpretation of PUE bands. Targets depend on climate, density and containment.
| PUE | DCiE | Rating | Typical context |
|---|---|---|---|
| ≤ 1.2 | ≥ 83% | Best-in-class | Hyperscale, free cooling, full containment, raised setpoints |
| 1.2 – 1.5 | 67 – 83% | Efficient | Well-run enterprise, contained aisles, economisers |
| 1.5 – 2.0 | 50 – 67% | Average / legacy | Traditional raised floor, partial containment |
| > 2.0 | < 50% | Inefficient | Small server rooms, no containment, oversized cooling |
DCiE (Data Center Infrastructure Efficiency) is simply the reciprocal of PUE expressed as a percentage.
Typical PUE by Facility Type
| Facility type | Typical PUE | Notes |
|---|---|---|
| Hyperscale cloud | 1.1 – 1.2 | Custom design, free/evaporative cooling, 27 °C+ supply |
| Modern colocation | 1.3 – 1.5 | Containment, economisers, efficient UPS |
| Enterprise data centre | 1.5 – 1.8 | Mixed-age cooling, partial containment |
| Legacy / converted room | 1.8 – 2.5 | CRAC oversizing, recirculation, no containment |
| Small server / comms room | 2.0 – 3.0+ | Single split AC, no airflow management |
Where the Overhead Power Goes
Approximate split of non-IT (overhead) power in a typical air-cooled facility.
| System | Share of facility power | Comment |
|---|---|---|
| IT equipment (useful) | 50 – 70% | Servers, storage, network — the denominator of PUE |
| Cooling & HVAC | 20 – 40% | CRAC/CRAH, chillers, pumps, fans — the biggest overhead lever |
| Electrical losses | 5 – 12% | UPS double-conversion, PDU, transformers, switchgear |
| Lighting & ancillary | 1 – 3% | Lighting, security, BMS, small power |
Cooling is almost always the largest overhead — containment, raised setpoints and free cooling give the biggest PUE improvement.
PUE → DCiE Conversion
| PUE | DCiE | Overhead share |
|---|---|---|
| 1.1 | 91% | 9% |
| 1.2 | 83% | 17% |
| 1.3 | 77% | 23% |
| 1.5 | 67% | 33% |
| 1.8 | 56% | 44% |
| 2.0 | 50% | 50% |
| 2.5 | 40% | 60% |
Frequently Asked Questions
What is a good PUE value?
Hyperscale facilities achieve 1.1–1.2 with custom designs and free cooling. A well-run enterprise data centre lands around 1.4–1.6, while older or small server rooms often exceed 2.0. There is no universal "good" number — it depends on climate, IT density and how much containment and free cooling you can deploy. The most useful target is a measurable improvement against your own baseline.
Why use annualised PUE instead of a single reading?
A single-moment PUE swings with outdoor temperature and IT load — cooling overhead is far lower on a cold night than a hot afternoon. The Green Grid and ISO/IEC 30134-2 define PUE as total facility energy divided by IT energy over 12 months, which removes seasonal bias and makes sites comparable. Use instantaneous PUE only as a planning snapshot.
What is the difference between PUE and DCiE?
They describe the same efficiency from opposite directions. PUE = total ÷ IT (lower is better, ideal 1.0). DCiE = IT ÷ total × 100% = the reciprocal of PUE as a percentage (higher is better, ideal 100%). A PUE of 1.5 equals a DCiE of 67%, meaning two-thirds of facility power reaches the IT equipment.
How do I lower my PUE?
Cooling is the biggest lever: deploy hot- or cold-aisle containment, raise supply-air setpoints toward the ASHRAE recommended upper band, add economiser/free cooling, and right-size CRAC/CRAH units. On the electrical side, eco-mode UPS and efficient transformers cut conversion losses. Blanking panels, brush grommets and removing cable obstructions improve airflow at almost no cost.
Does PUE measure IT efficiency?
No. PUE only measures how efficiently the facility delivers power to IT — it says nothing about how useful the IT work is. A site can have an excellent PUE while running idle servers. Pair PUE with IT-side metrics (utilisation, performance per watt) and, for sustainability, with CUE (carbon) and WUE (water) for a complete picture.