Who Moved My PUE?

Julius Neudorfer

At last the U.S. Environmental Protection Agency has released the final version of the Energy Star for Data Centers program.

It is the first official iteration of program, which began in late 2007. Now that is has been finalized, those in the industry should read the fine print. The Green Grid's original efficiency metric, Power Usage Effectiveness (PUE), is defined as Total Facility Power/IT Equipment Power. The EPA has decided that it also will adopt the PUE term because it was gaining widespread acceptance in the data center industry. However, the EPA's version of PUE is not the same as The Green Grid.
The EPA originally started to use the term Energy Usage Efficiency (EUE) during the early part of the program and later decided to change it PUE because the industry had become familiar with and adopted PUE. However, while the EPA is now calling it 'PUE' in its final version of the program, it has intermixed terms and has re-defined its version as 'PUE = Total Energy / IT Energy.'

While most people use the terms 'power' and 'energy' interchangeably, they are not the same. Power is an instantaneous measurement (expressed in watts or kilowatts, 'KW'), while energy is power over time, (expressed in kilowatt hours, 'KWH').  It is particularly odd that the EPA would chose to redefine PUE.

Moreover, it complicates the issue further by defining 'Total Energy and IT Energy' are to be expressed at 'Source kBtu'-not KWH, presumably to have it fit into its existing model for evaluating other types of buildings, which use fuels such as natural gas and oil, whose 'Source Energy' is expressed in BTUs. Confusing, yes, but nonetheless, be prepared to deal with it since it will be the U.S. government's de facto standard for evaluating a data center's efficiency.

According to the EPA's Energy Star Performance Ratings Technical Methodology for Data Center:


Total Energy includes the annual energy consumption for all fuels at the data center. In many cases, the only energy consumption at Data Centers is electricity. However, it is important to capture any other fuel use (e.g. chilled water, natural gas), in order to evaluate the total energy performance of the facility. This practice is consistent with all EPA rating models.

Moreover, while the EPA may have complicated the 'Total Energy,' it may have oversimplified 'IT Energy,' defining it as


as the total amount of energy required by the server racks, storage silos and other IT equipment in the Data Center. For the purposes of ENERGY STAR, this should be measured at the output of the Uninterruptible Power Supply (UPS).

While the Energy Star for Data Centers program is a voluntary program at this point, the data center industry should take heed now and follow these developments closely. In any event, consider the inherent advantages of improving energy efficiency, even if it only for the obvious advantage of lowering operating costs. 

And also consider going back to college to get your PhD in physics, since just when you thought you understood the PUE of your data center, the EPA moved it.

Add Comment      Leave a comment on this blog post
Jun 8, 2010 3:06 PM Julius Julius  says:
Hi Jack, You are quite right that just providing energy to the IT does not ensure that "work" is being done. (unless you consider a 15 minute YouTube video of someones cat sitting on a couch work). The Green Grid actually put forth a metric in their White Paper #6 in late 2007 called: �DCP� Data Center Productivity = �Useful Work�/Total Facility Power It was discussed and described as part of their �long term strategic thinking�. This is still an open issue since ultimately the concept of what a data center�s �Useful Work� is, makes it a difficult if not impossible metric to calculate or to get any or all groups to agree on So I agree, the the quest to define "productivity" continues.. Until then, at least we can measure the physical efficiencies. Julius Reply
Jun 8, 2010 3:06 PM Jack Pouchet Jack Pouchet  says:
To do something efficiently implies an improved level of productivity. Yet moving a data centers energy use ratio (PUE) from something like 4 to the EPA's identified average around 2.1 has nothing at all to do with productivity. Just because energy is being delivered to and consumed by IT equipment doesn't mean a single bit (or byte) of useful work is being accomplished! We need to focus on reducing total energy consumption within the data center. The Energy Logic strategies http://www.emerson.com/edc/page/Energy-Logic-Actions.aspx are a good starting point towards saving energy while improving space, power, and cooling capacity AND assuming the same amount of "work" is being run through the data center will significantly improve productivity. PUE is not related to productivity - today. Stay tuned as The Green Grid is working on a productivity metric that may help address this issue. Reply
Jun 9, 2010 10:06 PM Dan Diesso Dan Diesso  says:
This was a wise move on the part of the EPA. Whenever instantaneous measurements vary dramatically, it is dangerous to characterize �typical� performance based on measurements for a particular moment in time. For instance, if you look at the �instantaneous� mpg readout from your cars onboard computer it varies substantially. It would not be very accurate to look at the readout in your Chevy Suburban while coasting down a hill and proclaim you�re typical fuel mileage is 50 mpg. The better way to determine your typical mpg of is to look at the amount of fuel used over a longer period of time and dividing that into the miles traveled to come up with a more accurate estimate of typical mpg. With workloads varying greatly in the data center you run into the same problem characterizing PUE based on instantaneous power readings for a particular moment. You are going to get much more accurate results by using accumulated energy used over a long sampling period to characterize and compare PUE. You can always divide the kWh over the amount of time in the sampling period to report the average kW load as well anway. Reply
Jun 10, 2010 2:06 PM Julius Julius  says:
Dan, You are right that it is energy is a more accurate way to represent the overall efficiency. The Green Grid's PUE has a known shortcoming - it is only an instantaneous snapshot. They did try to address this with their "Advanced PUE Level 1-3".. The EPA's 11-12 month average "energy" based provides a better representation... My concern is that there are now 2 different definitions of the same term - PUE. Julius Reply

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