Glossary
PUE
Power Usage Effectiveness, the ratio of total data center facility power to power delivered to IT equipment; lower is better, with 1.0 the floor and 1.1 a strong target.
The ratio of total facility power (IT + cooling + lighting + losses) to IT power (the GPUs, CPUs, networking gear, storage). A PUE of 1.5 means half a watt of overhead for every watt delivered to the servers; a PUE of 1.1 means a tenth of a watt of overhead. The theoretical floor is 1.0; nobody achieves it.
The industry has compressed PUE substantially over two decades. Enterprise data centers from 2010 averaged near 2.0; modern hyperscaler facilities (Google, Meta, Microsoft) publish fleet averages around 1.10 to 1.15. The improvement came from free-air cooling in temperate climates, hot-aisle / cold-aisle containment, higher-temperature operation (chips run hotter than they used to), and increasingly liquid cooling for high-density AI racks.
PUE has limits as a metric. It does not capture water consumption (see WUE), grid-mix carbon intensity, or embodied carbon in the buildings and chips. A facility can have a great PUE and a bad sustainability profile.