An Argument for kW Capacity to Measure Data Center Size

This audio was created using Microsoft Azure Speech Services

At a recent conference, I overheard a conversation between two data center operators that went something like this:

  • U.S. guy: “My big data center has 10,000 square feet of floor space.”
  • Beijing guy: “My data center is over 1,000 square meters!”

I’m thinking, whose data center is bigger?  What’s the conversion again between square feet and square meters?  3 ft = 1 yard.  1 yard is about 1 meter.  I thought the US was switching to the metric system back when I was in elementary school!  Oh, never mind.

  • U.S. guy: “I can support more than 300 racks.”
  • Beijing guy: “Hmmm,  I can support 250 racks.”

This is easy. U.S. guy has more racks.  He can hold more servers.

  • US guy: “My IT load is 1,800 kW.”
  • Beijing guy: “My IT load is 2,000 kW!”

Now I’m confused. Beijing guy has fewer racks but is using more electricity. I guess his data center is inefficient?

While it was entertaining to hear the two flex their data center muscles, the conversation shows how people speak different languages when it comes to data centers, depending on their perspective. And it got me thinking about the best metric to describe the size or capability of your data center. It’s not the number of racks, or the area of the data center.   It is the design capacity of your IT load in watts, whether kW or MW.

Here’s why. IT load, as measured in kW, closely correlates to the amount of IT equipment you can support. And the cost to build and operate a data center closely correlates to the kW of IT load capacity. The amount of space a data center occupies amounts to a surprisingly low percentage of the overall cost to build a data center.

Using the kW metric can be tough depending on your background and business. You can’t see kWs flowing, but you can see how many racks you have and how big a room is.

Here’s a set of thumb rules you can use to help make sense of things:

  • The easy one: To convert from square feet to square meters, drop a zero – which is the same as dividing by 10. Logically, you add a zero to go the other way.  This gets you real close.
  • For older data centers, assume an average IT load of 2 kW per rack.  For newer data center assume 3 to 5 kW/ rack.  You might say, “But I know several data centers running an average of 8 to 12kW per rack.”  This may be true, but don’t worry. If you’re talking to someone who is operating or planning a data center with such a high density, they will tell you about it without prompting.  No need for guessing.
  • 1 sq meter of data center space = 1 kW of IT load capability at low densities
  • 1 sq meter of data center space = 2 kW of IT load capability at medium to higher densities
  • For dedicated data center facilities, you might hear about the power capacity supplied to the entrance of the building as a metric. It’s hard to jump from this number to IT load capacity because of possible redundant feeds, engineering buffer and efficiency of the data center – PUE.  But, if there are no redundant feeds, cutting the number in half can get you somewhat close to the IT load capacity.  Efficient data centers will have a bigger IT load capacity, while less efficient ones will be lower.

The next time you catch me listening in on your conversation, please stick to kW of IT load.  It will make my life a lot easier.

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