PUE stands for Power Usage Effectiveness and is a measure of the energy efficiency of an IT environment such as a server room or datacentre. The PUE term and calculation was developed by the The Green Grid. The PUE formula is:
PUE = Total Facility Energy / IT Equipment Energy
The nearer the PUE is to Unity (1) the more energy efficient the datacentre. Some mega datacentre operators can achieve a PUE around 1.2. For a low PUE ratio high efficiency systems must be used including uninterruptible power supplies, air conditioning units, lighting and servers.