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Power usage effectiveness (PUE) is a measurement of the efficiency of data centers. It is determined by dividing the total power entering a facility by the power used directly for computing tasks, and is expressed in the form of a ratio. Ratios closer to one indicate a higher rate of efficiency. A ratio of one would be functionally impossible, as data centers do need power for some support operations.
At a data center, racks of servers perform a number of tasks. They need support like cooling to prevent overheating and keep the center functional, along with lighting for personnel to perform work in the center, and so forth. Some data centers also have attached office space, in which case the power used for that area may be subtracted from the calculation used to find the power usage effectiveness to avoid skewing the numbers, as the energy used in an office doesn't have a direct bearing on the efficiency of the data center itself.
There are several ways to measure the amount of power fed to a data center. It is possible to take a reading directly off the meter, or from the power distribution packs used inside the center to send electricity to servers and support equipment. With this information in hand, technicians can measure the energy directly used by the servers for computing activities. Some data centers take measurements throughout the year at random to achieve a more balanced ratio, rather than taking a single set of measurements and using this as the basis for a power usage effectiveness calculation.
An unusually high power usage effectiveness ratio can suggest that a data center is not very efficient and steps should be taken to reduce its energy load. Low ratios indicate that data centers are using appropriate measures to control energy use, although more innovations may lower energy use even further. The use of an objective ratio can help companies measure their progress; as a data center performs better, the ratio drops. Effective consumption controls can result in an overall reduction in the amount of power used, bringing the ratio as low as possible.
Some information technology firms make their power usage effectiveness data publicly available and can discuss the techniques they use to calculate it, for members of the public who are interested. This can be more common among companies with a desire to project an environmentally friendly image. To demonstrate their commitment to environmental responsibility and a reduction in resource use, they can show the various techniques they use to reduce and control the use of natural resources.