Established by The Green Grid, Power Usage Effectiveness is a calculable measurement used by many.
By Patrick McLaughlin
In November we reported on some steps the United States Environmental Protection Agency (EPA; www.epa.gov) is taking to encourage energy-efficient data center operations. (See “Adopting best practices for efficient energy use,” November 2009, p. 21.) Since the time that issue was published, the EPA has reconsidered the primary means by which it will measure energy efficiency for its upcoming EnergyStar program for data center facilities.
The agency originally planned to establish and use a measurement it called Energy Usage Effectiveness, or EUE, rather than the Power Usage Effectiveness, or PUE, metric established by The Green Grid (www.thegreengrid.org). In a conference call last September, the EPA explained that it opted to develop EUE to quantify overall energy consumption, including natural gas, diesel, and other fuels, rather than simply power consumption. In addition, the EUE calculations would be made based on source energy rather than site energy, in keeping with its other EnergyStar programs.
Power Usage Effectiveness and its reciprocal, Data Center Efficiency, compare the total power consumed by a data center facility with the amount of power its IT equipment consumes.
In a follow-up conference call in November 2009, the EPA said it would in fact use PUE as the metric for the data center EnergyStar program. The agency made this decision based largely on feedback it received after announcing the EUE metric. Representatives of the EPA stated that this feedback included the pointing out that PUE does allow for the calculation of total energy use, not just electricity use.
Calculating PUE
PUE is a simple formula to measure complex consumption. That simple formula is total facility power divided by information technology (IT) equipment power. When The Green Grid proposed the use of PUE in early 2007, it also proposed the use of its reciprocal—IT equipment power divided by total facility power, which the organization dubbed Datacenter Efficiency (DCE). The next year, The Green Grid revised the DCE measurement to DCiE, for Datacenter Infrastructure Efficiency. Its calculation is 1 divided by PUE, or IT equipment power divided by total facility power x 100%.
By virtue of the calculation, the closer a PUE gets to one, the more efficient the data center’s IT equipment is. The EPA collected energy-consumption data from 121 data centers in its initial information gathering for the EnergyStar program and reported those facilities’ PUE ratings ranged from 1.25 to 3.75, with an average rating of 1.91. More than 30 of the participating data centers had PUEs between 1.51 and 1.75; more than 20 rated between 1.76 and 2.0; and between 15 and 20 facilities had a PUE between 2.01 and 2.25.
In the 2008 Green Grid white paper that presented the DCiE metric, authors Andy Rawson, John Pfleuger, and Tahir Cader explained, “Ideally, a PUE value approaching 1.0 would indicate 100% efficiency (i.e. all power used by IT equipment only). Currently there are no comprehensive data sets which show the true spread of the PUE for data centers.”
Getting close to 1
Of the 121 data centers analyzed by the EPA, only one achieved a 1.25 rating. According to Ron Mann, director of engineering with HP’s rack and power infrastructure group, another government agency is zoning in on low PUE. In January the Department of Energy (DOE; www.energy.gov) announced $47 million in grants to organizations for the specific purpose of supporting the development of technology to increase energy efficiency in IT and telecom facilities.
HP received a $7.4 million grant from the DOE to develop a modular data center with integrated power, air conditioning, and distributed-energy systems that promises to reduce energy requirements. Mann commented that a requirement of the grant is that the technology developed results in a PUE that does not exceed 1.25. “The closer you get to 1, the more efficient you are,” he said. “To do that, you’ve got to put [the IT equipment] in a controlled environment.”
Yahoo! got nearly 10 million of those DOE grant dollars to passively cool a data center it is currently building in Lockport, NY. When the DOE announced the $9.9 million grant to Yahoo!, it described the undertaking as follows. “The integrated building design, including the building’s shape and orientation and the alignment of the servers within the building, allows the data center to use outside ambient air for cooling 99 percent of the year. The relatively low initial cost to build, compatibility with current server and network models, and efficient use of power and water are all key features that make this data center a highly compatible and replicable design innovation for the data center industry.”
The EPA addressed the use of airside economizers in its November conference call. To put into context the EPA’s position on the use of economizers, the agency was responding to suggestions that a facility applying for the EnergyStar rating should be rewarded with additional point(s) for using economizers. The EPA’s explanation in response was facilities that properly use economizers will in fact achieve lower energy use and therefore a lower rating. The agency emphasized the word “properly” in addressing that issue.
Refining the metric
In its 2008 white paper, The Green Grid recognized some challenges in calculating PUE and DCiE, including the integration of cooling elements into some of the latest IT equipment. “These technologies blur the lines between what has traditionally been a clear delineation between facility equipment and IT equipment,” the paper said.
More recently the group published a white paper entitled “Use and public reporting guidelines for The Green Grid’s infrastructure metrics (PUE/DCiE).” That paper addresses the manner in which organizations publicly state their PUE and DCiE ratings.
Last year The Green Grid unveiled a proposed Data Center Productivity (DCeP) metric. The simple formula for its calculation is DCeP equals useful work divided by total energy consumed. For those who will try to calculate the metric using real numbers, it will be significantly more complex. Recently the organization posted a presentation on its Web site updating the work it has put into that effort; that work is ongoing.
Originally put forth just a few years ago, the Power Usage Effectiveness rating has taken hold as a popular metric by which data centers count their energy efficiency. The Green Grid continues to refine the metric and its use.
Patrick McLaughlin is chief editor of Cabling Installation & Maintenance.