Julius Neudorfer is the CTO and founder of North American Access Technologies, Inc. and has been involved with designing data center infrastructure since 1987. He is also the founder of THINK8760.org which is focused on improving data center energy efficiency.
With any popular movie, there is usually a sequel. It looks like the rise in popularity of the Power Usage Effectiveness (PUE) metric has followed the same path. PUE has clearly become the most popularly quoted data center metric lately, perhaps in some cases if only as misguided marketing tool.
On May 17th the Data Center Efficiency Task Force issued a 14 page document entitled “Recommendations for Measuring and Reporting Version 2 – Measuring PUE for Data Centers” - also known as PUE 2.
While PUE was originally created by The Green Grid in 2008, it was adopted in early 2010 by the Data Center Efficiency Task Force, a group consisting of 7x24 Exchange, ASHRAE, Silicon Valley Leadership Group, the U.S. Department of Energy's Save Energy Now Program, the U.S. Environmental Protection Agency’s ENERGY STAR Program, United States Green Building Council, Uptime Institute and The Green Grid.
We'll hear thoughts from members of the task force shortly. But first, let's take a look at what's new.
The updated PUE version is meant to clarify and reiterate the updated measurement requirements of the PUE metric and presumably prevent "PUE envy." It reflects and reiterates the "Harmonizing Global Metrics for Data Center Energy Efficiency" definition of PUE which was released by the task force in February of this year.
Energy Rather Than Power
While it is still called PUE, it now specifies energy usage (expressed in KWH) not power (KW) as the primary basis for the metric. This still seems to be one of the sources of confusion to those using and quoting PUE numbers for their data centers.
There are four PUE measurement categories (0-3), of which three upper categories (1-3) specify annualized energy consumption as the basis for the calculations. Even PUE Category 0, which still allows power demand expressed in KW, now requires that the calculation be based on the highest power used by the facility, not just the lowest power measurement taken on a cold night when the chillers are off.
“PUE Category 0: This is a demand based calculation representing the peak load during a 12-month measurement period.”
This should help level the playing field of some seemingly exaggerated PUE claims.
Guidance on Measurement
The PUE 2 document is an enhancement and continues to follow the "3 Guiding Principles" and the "Harmonizing Global Metrics" document issued in February by The Green Grid and the Task Force and also adopted by some international organizations.
Moreover, PUE 2 includes more details on how to properly measure and calculate PUE for data centers located in a mixed use building, in particular building-supplied chilled water or condenser water.
One of the significant changes in PUE 2 which may be easy to overlook, is the following caveat regarding the requirement to reference the PUE category when proclaiming a data center’s efficiency.
“When publishing PUE, the category must clearly be indicated using a subscript e.g. PUE0, PUE1, PUE2, PUE3. A PUE reported without the subscript is not considered to be in compliance with these recommendations.”
It would seem that very few (if any) of those who have previously made public proclamations of their PUE results have indicated which PUE category methodology and protocols they used to derive their claimed results. Going forward they will hopefully include the category in future PUE announcements. It should be interesting to see how many organizations will adhere to this requirement in any new crop of PUE announcements.