Data Center PUE – What Does It Mean?

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Jerry Gentry is the VP, IT Program Management at Nemertes Research

We are all trying to make our data centers green.  It’s part of being a good corporate citizen as well as being a good citizen of the earth.  When you look at those places in the corporate infrastructure that eat up the most energy, the data center usually fall close to the top of the list for the IT department.  So executives or clients are asking you what you are doing to spend less on energy. Your data center Power Usage Efficiency (PUE) provides insights into your efficiency efforts.

Like they say in the P90 workout, “You can’t tell where you’re going unless you know where you’ve been.”  Before you can start showing how effective the changes in investment and operating processes are a baseline must be established. That baseline must be repeatable and verifiable. 

That is what the Green Grid is helping to do.  They have provided some consistent measuring methods that a data center manager can use to establish a baseline and then record the impact of infrastructure and operating process changes to power utilization.  The first calculation is called Power Usage Efficiency (PUE).  It measures how effective your data center is in using the input power.  This ratio of power available to power used will yield a factor of greater than 1.  The larger the number the less efficient your utilization is.

The second calculation is Data Center Infrastructure Efficiency (DCiE).  It is the inverse of PUE and is the ratio of what you use versus the total power available.  The factor for DCiE will always be less than 1.  The closer to 1 the better. 

Fortunately, there are online calculators that will take your input and show you the outcomes and how your rating stacks up.  They do require that you know at least those two parameters – total power available and total utilization, both in KW. 

There is importance to PUE and DCiE measures that go beyond evaluating your in-house data center and helping validate your green initiatives.  A common question of data center managers facing the life cycle management of their infrastructure is whether they should build or buy.  Understanding your efficiency in managing a data center is one of the key decision criteria for making that choice.  But, it is more than just the number.

If you are able to establish your PUE and the ratio comes out as 1.4.  You may have a green initiative to move that to 1.3 or better.  To get to 1.3 you may have a number of investments to make, including updating the power distribution units (PDU’s) or the computer room air conditioning units (CRU’s).  It might mean a renovation of cabling under the floor tiles to improve airflow or redressing power cable runs or restacking your equipment for more optimal heat dissipation.  Each of those activities will carry a cost in time and materials.  They also indicate some amount of system down time might happen, which also has a cost.  

Understanding the real cost in gaining power efficiency leads to a rational conversation about the best investments to make.  Few enterprises have unlimited investment funding and must prioritize the next dollar spent.  Having a quantitative analysis based on a standard calculation, like PUE, will add credence to your analysis and help you better compete for those scarce dollars.  

At the end of the day we want to be good citizens all around, which means finding that balance between spending scarce investment dollars and achieving a greener footprint.  Starting the dialog with facts is always preferred.

Jerry Gentry is the VP, IT Program Management at Nemertes Research

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