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.