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Calculating Space and Power Density Requirements for Data Centers

White Paper 155 Summary     Revision 0     By Neil Rasmussen

The historic method of specifying data center power density using a single number of watts per square foot (or watts per square meter) is an unfortunate practice that has caused needless confusion as well as waste of energy and money. This paper demonstrates how the typical methods used to select and specify power density are flawed, and provides an improved approach for establishing space requirements, including recommended density specifications for typical situations.

There are four major problems with the historic practice of describing power density in terms of watts per square foot or watts per square meter.

1.    What is included in the area calculation, or how it relates to the number of IT cabinets or devices is not defined.
2.    What is included in the power calculation is not defined.
3.    It provides no information about the variation in power across a population of IT cabinets; is it a peak number? An average over area? An average over time? Or some other value?
4.    It is not clear how this number is used in a data center that has a changing growth plan or is modular or built out over time.

In principle, the first two problems could be improved by establishing standard definitions for power and area. However, the third and fourth problems are very important and cannot be solved by improving current definitions. A better approach to specifying density considers the reality that IT power varies among cabinets as well as over time, and comprehends the issues of modularity and growth. These problems can be summarized with the following statement:

Specify too low a density and performance becomes unpredictable with various overload and overheating problems occurring; specify too high a power density and first cost and operating expenses are needlessly increased.

Other key takeaways include:

•    It is much costlier to deploy IT below the data center design density, than to deploy above the design density. This is true because the cost of space per unit of IT is always much lower than the cost of power and cooling per unit of IT. Given the fact that the actual density of IT equipment in a data center is difficult to predict in advance, this leads us to a critical key conclusion:
•    When the density of IT equipment is uncertain, a data center should always be constructed for a design density less than the mean expected value of IT density.
•    A well-designed data center, when filled to power and cooling capacity, is expected to have spare or unutilized IT space.

The new approach to the specification of space requirements and power density has four key features:

•    The unit of physical space in the density specification is the IT cabinet, NOT floor area. Floor area is determined during the design as an output of the process using per cabinet power and other factors. 
•    The specification is hierarchical and modular, so that different rooms and zones can have different density requirements. 
•    The specification comprehends that IT cabinets within data centers have different power requirements, and that these requirements may not be well-defined in advance. 
•    The specification comprehends that IT equipment cabinets may have power requirements that vary with time.

The paper goes on to explain these features in detail. And then it provides sample density specifications (as well as a useable worksheet) such as the one pictured below (see Table).

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