Power usage effectiveness [/ˈpaʊər ˈjusɪʤ ˈifɛktɪvnəs/] n - Power usage effectiveness (PUE) is the efficiency metric every data center quotes:
A PUE of 1.0 would mean that every watt drawn at the Where the Grid Comes In reaches the IT load, with nothing spent on overhead. Real facilities land between roughly 1.1 and 2.0; the remainder goes to cooling, to lighting, and to the losses within the power chain itself.
Where the Overhead Hides
- Cooling [/ˈkʌːlɪng/] n dominates, usually the largest non-IT consumer.
- Conversion losses [/ˈkɒnvɛrsɪɒn ˈlɒssɛs/] n gather in the Uninterruptible power supply, in the transformers of the Medium-voltage switchgear and the Power distribution unit, and in every stage that changes voltage. Hence the close scrutiny given to double-conversion UPS efficiency.
How It Is Misused
- It is a ratio, so adding IT load can improve PUE while saving no energy at all.
- It is blind to the source of the electricity; a coal-fed PUE of 1.1 may be “worse” for emissions than a renewable PUE of 1.4.
- It says nothing of resilience. A fault-tolerant 2N site (see Redundancy topologies and the Uptime Institute tier classification) runs its UPS and The Standby Engine paths partially loaded, which inherently raises PUE.
A true PUE wants metering along the whole chain: at the Power distribution unit, the Remote power panel, the Busway distribution taps, and the Rack power distribution unit.
Part of The Engines of the Hall.