Power Distribution Losses: Downstream of the UPS, these losses are sometimes ignored; typically they can represent three to five percent for 'copper' (I2R) losses, and sometimes up to five to seven percent if the PDUs contain a transformer. In fact, the Environmental Protection Agency (EPA) even found that a majority of the 120 original Energy Star for Data Centers program participants did not have the capability to measure the IT load 'at the plug.' As a result, the EPA is willing to accept the UPS output as the IT load, if more accurate IT load data is not available.
Generator: The block heaters in backup generators require constant power to ensure the generators start quickly.
Support Areas: The power for required HVAC ventilation and lighting for related support areas such as electrical, battery and UPS rooms, as well as telecom/datacom entrance rooms.
Condenser or Chilled Water: If there is no dedicated condenser or chiller plant, and it is supplied from a third party or from within a mixed-use building, its energy value has been overlooked by the person making the PUE calculation. Also, some manufacturers of data center containers that use chilled water and expect conditioned power from an external UPS have aggressive PUE claims. In fact, in general, they should not be able claim a 'PUE' for their container only, unless they also provide all of the support infrastructure energy in the calculation, just like any other conventional site.
Fan Deck Power: The external DX condenser or glycol fluid dry coolers are many times excluded from measurements in existing sites, again particularly in mixed-use buildings.
Security Systems: Security and access control systems, as well as CCTV systems are often ignored, especially if they come from a dedicated power panel with separate backup power or are part of the main system in a mixed-use building.
Emergency Lighting Systems: They are (or should be) on their own dedicated backup power circuits and are often overlooked.
The Calendar: The time of year can lead to improper PUE calculation methodology. The original PUE calculation was based on a power reading, and many times it was taken as an instantaneous snapshot, under 'best case conditions' (i.e., coldest day of the year, running only on an economizer) and thus did not represent an actual 12-month average of energy usage. This is now a requirement and is part of the new Harmonizing PUE guidelines issued in February 2011 by The Green Grid, and adopted by the EPA, European Union and Japan.
Candor: If you are going to claim a very low PUE for its promotional value, then at least provide the basis of the calculation measurement and any related caveats or conditions (even if it is small print). It seems that there are almost always some losses that they think should have been included but were not because the marketing department said to ignore it since it would worsen its spectacular PUE claims.
The Green Grid has specific PUE protocols and if they are followed should provide a fair overall assessment of the energy efficiency of the data center's infrastructure. Toward that end, I thought that I would go right to the source for comment on my views on the 'sins of omission' for PUE claims.
In an email exchange with Mark Monroe, executive director of The Green Grid, I received this comment:
The Green Grid has worked to drive industry alignment of PUE through a consistent, repeatable measurement strategy and reporting processes. These efforts represent a significant step in advancing data center energy and resource efficiency on a global scale. Establishing a common understanding of efficiency metrics within the industry will help generate the necessary dialogue among data center owners and operators to improve efficiencies and reduce energy consumption within these facilities.
The Bottom Line
In the short term, the race to be the greenest data center may produce some overly ambitious PUE claims. Some of these, were they to be fully examined, might prove to be exaggerated. Nonetheless, the long-term outlook indicates that there is now a common awareness and consensus in the industry that making data centers more energy efficient has many direct economic, as well as political, benefits to the owners and users, as well as the planet in general.
So next time someone claims that their data center has achieved a PUE of 0.9, ask them if they were part of the team of scientists that developed cold fusion.