Watts Per Square Foot
Data Center Power Series – Part 4
The rapid evolution of data centers and data center infrastructure technology has brought about many definitions and opinions regarding industry terms, capacity calculation methods, and best practices used in data center environments. This document is developed from a collection of definitions and/or opinions on the topic of watts per square foot (WPSF) calculations and is intended to present a clear analysis of the three primary calculations used by the data center community.
Watts per What?
At the core of this generally confusing topic is the basic question of what physical space measurement method to use when calculating square footage for the WPSF calculation.
The WPSF calculation is simple:
Available Watts ÷ Square Feet = Watts per Square Foot (WPSF)
Clearly, the value used for Square Feet can significantly change the result of the WPSF calculation. As illustrated in Figures 1, 2 and 3 below, the Square Feet value varies greatly depending on the measurement method used. To determine the correct method, first understand the reasoning behind each. Once a choice is made, ensure the same method is used when calculating WPSF for apples to apples comparisons between facilities.
The three most commonly used space measurement methods currently in use are: Room Envelope, Production Space, and Rack Footprint. Further adding to the confusion, the methods are sometimes referred to by different names, however, for the purposes of this document (and the ongoing quest to help standardize industry terminology) we’ll stick to these simple and descriptive names.
Room Envelope
Room Envelope is simply the full internal dimensions of an enclosed and structurally defined data center space. This can be a single room, or multiple rooms connected by open hallways and should include the open hallway area, entry ramp area, etc. The idea here is to include all open air space requiring cooling and environment control as shown in Figure 1 below.
Why use Room Envelope? While this method may not provide the best WPSF data for actual equipment density, it can be very useful for general space and capacity planning benchmarks, non-containment cooling calculations, and as a baseline value for general facility comparison purposes.
Production Space
Production Space uses the total square footage for racks including clearance area (typically 48 inches front and 36 inches rear) plus infrastructure equipment (and associated clearance area) directly related to the support of racks and devices.
Why use Production Space? The value derived from Available Watts ¸ Production Space yields a WPSF value for Wattage distributed across the equipment racks plus the associated supported equipment. This method is particularly useful where the additional power requirements for support equipment are of specific interest such as when cooling capacity is a concern.
Rack Footprint
This method uses the total square footage for racks and rack clearance area only (typically 48 inches front and 36 inches rear). Using Rack Footprint for the Square Footage value provides a WPSF value specific to rack equipment density only.
Why use Rack Footprint? This method is often used where rack equipment density is important regardless of room size and support equipment power requirements. For example, in cases where support equipment is separated from the rack areas by containment or other means. This method yields the highest WPSF value as shown in the example calculations below.
Example Calculations
As shown in the sample calculations below using a common Available Wattage value, say 100,000 Watts, the square footage value used (from Figures 1, 2, and 3 below) significantly changes the WPSF result.
- Room Envelope: 100,000 Watts ¸ 1280 Sq ft = 78 WPSF
- Production Space: 100,000 Watts ¸ 884 Sq ft = 113 WPSF
- Rack Footprint: 100,000 Watts ¸ 416 Sq ft = 240 WPSF
CONCLUSION
Using the information presented here, the appropriate value for square footage can be determined for most applications. Most importantly, the calculations between facilities and/or vendors can be aligned for consistency when comparing values and determining actual capacity available.
