Thursday, July 16, 2009

Adding a Geographic Element to PUE Calculations

The PUE metric has become one of the most significant metrics for measuring the gross efficiency of a data center. As data center operators boast of PUE numbers that approach the optimal rating of 1.0, it's often difficult to separate out environmental or regional factors.

Is a PUE of 1.5 in Phoenix better or worse than a PUE of 1.4 in Seattle?

It depends. In absolute numbers, the lower PUE provides an indicator of the most efficient facility. However, achieving a PUE of 1.5 in Phoenix is much more difficult than an equivalent or slightly lower number in Seattle because Phoenix is so much hotter and requires more air conditioning. Moving data centers to cooler locations helps the PUE rating, but sometimes data centers need to be located in a specific city or region. How can you compare PUE values in regions with different environmental conditions?

One possible approach is to add a geographic compensating factor:

gPUE = G * PUE

The geographic compensating factor G would be determined by The Green Grid or other trusted body based on compiled weather data. Ideally, this could be calculated empirically through a formula using data maintained by the U.S. Department of Energy (refer to this blog link for information on that data and a free tool to visually represent that data).

This approach would allow somebody to measure the technical innovation of a given facility while providing an adjustment to account for geographic disparities in temperature, wind, solar loading, etc. It's not a perfect solution (since some cooling optimizations might not work in cooler or hotter climates), but it provides some measure of equalization to facilitate more equitable comparisons between PUE claims in different locations.


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