Who Moved My PUE?

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.