PUE (Power Usage Effectiveness) measures a data center's energy efficiency: the ratio of total energy consumed by the site to the energy actually used by the servers. A PUE of 1.0 would be perfect; a good modern data center targets 1.1 to 1.4.
The closer PUE is to 1, the less energy is “lost” in cooling and losses — hence lower environmental impact and often better-controlled electricity cost. It's a green-data-center criterion.
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