Getting to know your Power Usage Effectiveness (PUE).

If the subject of power efficiency metrics isn’t a topic of conversation in your operations today, it will be soon. Because nothing is more important than understanding its role – and the role your rack PDUs can play in providing the precise data you need – when improving your overall data center efficiency.

One of the key building blocks in any effort is actionable information. The goal, therefore, is to maximize the amount of useful computational work by IT equipment for total energy consumed by the equipment itself and the infrastructure that supports the equipment.

The most commonly used metric for this is The Green Grid’s Power Usage Effectiveness, or PUE:

PUE = Total Facility Power / IT Equipment Power

"Total Facility Power" in this equation is all the power required to operate the entire data center, including the IT equipment items: servers, storage, network equipment and other IT equipment; and the support infrastructure items: CRAC units, fans, condensers, UPS and lighting. "IT Equipment Power" is the power required to operate the servers and IT equipment alone.

Theoretically, PUE can range from 1.0 (where all the power is consumed by IT equipment only) to infinity. So a PUE = 2.0 means that IT equipment is consuming 50% of data center power. Another commonly used metric is Data Center Infrastructure Efficiency (DCiE), which is the inverse of PUE. Since they are derived from the same data, there is no substantive difference in the measurement or its usage.

A recent study by Lawrence Berkeley National Laboratory of data center power allocation showed an average data center PUE of 2.18, and a similar result was found by EYP, with the following allocation:

Three levels of PUE

The Green Grid defines three levels of PUE: Basic or Level 1, Intermediate or Level 2, and Advanced or Level 3. Many industry analysts recommend measuring IT power consumption at the Intermediate, Level 2; i.e., at the PDU level. While it is true that PDU-level power consumption will provide the denominator needed to calculate PUE, this information alone is unlikely to be sufficient to drive the best efficiency improvement decisions. Regardless of the PUE level you choose to employ, the best practice is to gather data over a time period of “typical” power usage to ensure that the peaks and valleys have been captured in calculating your PUE to establish a baseline and to track your improvements.

Why Advanced, Level 3 PUE?

An improved (lower) PUE can be misleading since it can result from inefficiencies in the power consumed by IT equipment, which merely increases the denominator. A lower PUE is generally better than a higher one, but it is possible to implement measures that reduce data center energy consumption yet actually increase your PUE. For example, if you were to replace older, less efficient servers with more efficient ones, eliminate ghost servers, turn off servers that were idle during the night, or employ server virtualization, the net result is power reduction but your PUE would actually increase. The detailed IT load data from Level 3 provides the granularity of information to reduce energy consumption, not just improve the PUE metric. Clearly, the PUE (and its inverse DCiE) becomes a more useful beacon once you have built efficiency into the performance of the IT equipment. And to do that you will want the granular power usage data for the Advanced, Level 3 PUE metric. Then you can attack the numerator and squeeze inefficiencies out of the infrastructure.

As a member of The Green Grid, Raritan has embraced the effort to increase data center energy efficiency and has developed a number of solutions that help IT administrators and facilities managers monitor equipment, collect data and identify areas that can help you improve your PUE.

Learn more about The Green Grid

Learn more about Raritan’s power management solutions