In 2007 The Green Grid introduced power usage effectiveness (PUE) as a metric to better understand and improve the energy efficiency of existing and new data centers. Until this development there was no broadly accepted method of evaluating the impact of conservation measures on energy efficiency. Last June PUE was designated by the U.S. Environmental Protection Agency as the basis for the Energy Star label. Today PUE is the principal efficiency metric used by the IT industry to compare the relative efficiency of data processing facilities.
The Green Grid actually defined two terms in 2007, PUE and its inverse, data center infrastructure efficiency (DCiE).
PUE is defined as total facility power/IT equipment power, and DCiE is defined as 1/PUE x 100 percent. In these definitions, IT equipment power is the entire power load associated with all the IT equipment including processing, storage, and network hardware, plus equipment used to monitor and control the data center. Total facility power is all the power dedicated solely to the data center, measured at the utility meter, so it includes IT equipment power plus everything else that moves the needle on the electric meter such as UPS systems, switch gear, PDUs, batteries, the cooling system, data center lighting, and other miscellaneous loads and losses. By definition, perfection is achieved when PUE and DCiE reach 1.0, but as a practical matter PUE will always be a number greater than 1.0 and its inverse, DCiE, will always be less than 1.0.
The broad acceptance of PUE is surprising given its obvious deficiencies as a useful measurement tool, especially when comparing one data center with another. One problem with PUE is that the acronym makes no sense. PUE stands for “power usage effectiveness” but might as well mean “pig with a unicorn horn and elephant trunk,” shown in Figure 1, because neither cluster of words meaningfully describes what is going on in a data center.