Green Grid offers choices for measuring data center efficiency

Group breaks PUE into four categories, ranging from quick and flexible to granular and precise

One of the greatest strengths of the PUE metric, the industry standard for measuring data center energy efficiency, is its simplicity: Calculate how much energy your data center is consuming overall, then divide that number by how much energy your IT equipment alone consumes. The resulting number is your PUE, which you can then compare against your previous scores or those of rival data centers. The lower the number, the more efficient your data center and the greater the bragging rights.

At the same time, the simplicity has its shortcomings. For example, it gives operators much flexibility as to where to measure consumption -- at the PDU or at the point of connection of IT devices -- as well as how often to take measurements. Thus, two similarly sized data centers might boast nearly identical PUEs, even though one has a highly efficient DC-based power distribution system and measures PUE continually, while the other has a traditional AC-based system and measures PUE once per month during off-peak hours.

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In an effort to overcome this drawback, The Green Grid has unveiled four categories of PUE, ranging from Category 0 to Category 3, in a new white paper, "Recommendations for Measuring and Reporting Overall Data Center Efficiency" [PDF]. With each level, the measurements become more granular and the results more precise. Thus, a data center operator may choose to go with Category 0, which requires the least effort and fewest resources -- but then those results won't be viewed in the same light as a rival's Category 3 PUE figure.

Specifically, measuring your Category 0 PUE would entail figuring out your data center's total peak load at the utility meter over a 12-month period, typically reported as demand kW on a utility bill. You would then divide that figure by the demand reading of your UPS system's (or systems') output as measured during peak IT utilization.

Notably, this approach ignores the use of power sources other than electricity, such as natural gas. "As this is a snapshot measurement, the true impact of fluctuating IT or mechanical loads can be missed. However consistent measurement can still provide valuable data that can assist in managing energy efficiency," according to the report.

For Categories 1, 2, and 3, you first gather total energy by adding your peak-load kWh readings from a 12-month period to how much natural gas or other fuel your facility consumes in over a year, converted into kWh. From there, the main difference among the remaining categories is where you measure your IT load: at the UPS system output, at the output of PDUs supporting your IT load, or most accurate, at the point of connection of the IT devices to the electrical system.

"This [final] measurement method provides the highest level of accuracy for measurement of the IT load reading by removing all impact of losses associated with electrical distribution components and non-IT related devices, e.g., rack mounted fans, etc.," according to The Green Grid.

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Additionally, The Green Grid has devised a way to weight PUE calculations by what types of energy are being used. For example, suppose one data center only uses electricity for everything, including chilling its own water, whereas a second data center is identical except it purchases district chilled water. In the latter case, the data center operator would determine how much energy went into chilling the amount of water his or her facility used, then multiplying that number by 0.3.

These new categories for PUE, along with the other recommendations for the metric, represent important steps toward making PUE more relevant and useful. The Green Grid still has work to do, though, in figuring out more granular ways of measuring PUE at data centers that are part of a share site, as well as factoring in data center workloads and tier levels.

This story, "Green Grid offers choices for measuring data center efficiency," was originally published at InfoWorld.com. Follow the latest developments in green IT and read more of Ted Samson's Sustainable IT blog at InfoWorld.com.

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