PUE is DEAD: The Case for Performance Per Watt

Mark Davidson, Sustainability Officer for JouleX, an innovator in enterprise energy management systems for data centers, distributed office environments and facilities.

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Power Usage Effectiveness (PUE) has been called "The Holy Grail" of data center energy metrics so often that we actually found it impossible to find out who coined the term. As time, technology and sustainability efforts evolve, the PUE metric is no longer the stopping point for energy efficiency measurement, but it has become just one more piece in the larger picture.

What does PUE do? It measures how much of the energy entering a data center facility is used to power the computing devices within, versus the amount used for cooling and overhead of the facility. That’s it.

Realities of Measurement

In an ideal world, the PUE is 1.0, which means that 100 percent of the energy is used by the computing devices in the data center. Since 1.0 is an impossible-to-achieve ideal, the standard goal for most data centers is a PUE of less than 2.0, which means that for every 1 watt of energy used by the computing devices, an additional 1 watt is used for facilities overhead such as air conditioning and lighting.

At first glance, this is a very direct metric. It’s easy to understand and easy to follow. The problem with relying solely on PUE is that it in no way measures the efficiency of the IT devices themselves.

Other Metrics Available

Data center managers today are under pressure to deliver increasingly higher energy efficiency and lower costs. In order to understand the true efficiency levels and progress toward enterprise sustainability goals, managers MUST have access to an accurate performance measurement of each device in the facility. Other available energy metrics include:

Another Measurement - Performance per Watt

There are, however, issues with these other metrics. CADE is a McKinsey metric and is very costly to implement. By the Green Grid’s own admission, DCP and DCeP are very difficult to measure and implement. The measurement recommended by JouleX is Performance per Watt (PPW).

What’s the difference? Like PUE, Performance per Watt is a very straightforward and easy-to-implement solution. The PPW metric measures the actual energy efficiency of every device in the data center and how it is used.

The PPW approach uses a relative performance indicator for each individual asset. This indicator is calculated by the types of hardware and capabilities learned from an asset inventory of that device. This Performance Indicator (PI) is a simple measurement for getting relative performance of the device in question.

At JouleX, we use a derivative of SPECmarks. However, any performance metric can be used as long as it is incorporated into all your measurements. The formula for PPW is as follows:

(PI * Avg Device Utilization / Watts ) * 100

Let’s take a look at two examples:

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When the device is at maximum efficiency, the PPW number is higher. The lower the PPW number, the more power that device is wasting. What can the PPW measurement tell you?

When combining PI with live utilization of an asset, along with real-time energy draw of that asset, you get a simple process for measuring PPW. Being able to measure Performance per Watt will help data center managers identify devices that are wasting energy.

And, as we’ve always said: ”You can’t manage what you can’t measure.”

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