Power Usage Effectiveness (PUE) is the accepted metric for measuring and reporting data center energy performance. But with so much interest in green data centers in recent years there was a need to create greater consistency and clarity in how power usage effectiveness was measured and reported. Readers of this blog may recall earlier stories about the use of PUE, Green IT, and scorecards for Internet data centers and the difficulty in comparing one data center to another.
To address the issues with PUE the Taskforce that gave us the original version of the PUE met again and in May of 2011 released “Recommendations For Measuring and Reporting Overall Data Center Efficiency Version 2 – Measuring PUE for Data Centers” (Download PUE Version 2) clarifying the metric.
PUE Version 2 does a couple of very important things.
- It specifies that PUE is to be a measurement of IT energy consumption expressed in kilowatt hours not electric demand measured in kilowatts.
- It defines four measurement categories PUE 0-3 using 12-month periods
- It requires the PUE Category to be specified whenever publishing PUE
Here is a simplified explanation of each PUE Version 2 Category and the PUE Version 2 diagram from Mission Critical Magazine to help illustrate where in the data center electrical system the measurements are to be taken. Of course this is not a substitute for reading the Taskforce Recommendations since it includes additional considerations for the type of data center and the electrical energy sources used.
PUE Category 0 (PUE0): Is a demand based calculation using the peak load measured at the UPS output in kW during a 12 month period and the total data center power measured at the data center boundary. This is the least preferred method for measuring power usage effectiveness and is only intended to be an entry level method so that more can participate.
PUE Category 1 (PUE1): Is a consumption based calculation using the total cumulative energy consumption at the UPS output in kWh over a 12-month period. Because PUE1 accounts for fluctuations in IT loads it is a more accurate picture of performance.
PUE Category 2 (PUE2): Is also a consumption based calculation using the total cumulative energy consumption in kWh at the output of the power distribution units (PDU) supporting the IT loads. PUE2 is a more accurate performance measurement because it is taken closer to the IT loads eliminating the losses from the UPS, switches and other electrical system components
PUE Category 3 (PUE3): Is also a consumption calculation of cumulative energy over a 12-month period in kWh but it uses measurements taken directly at the point of connection of the IT loads to the electrical system. This is the most accurate measurement of performance since it removes all losses from the electrical system and non-IT loads.
As I said, there is a lot more to this than I am covering here but that doesn’t mean it is terribly complicated either. In fact you can find a fantastic summary written by Julius Neudorfer in the November/December issue of Mission Critical Magazine. In the article Neurdorfer also mentions the importance of PUE Version 2 along with other data center performance metrics on water usage effectiveness (WUE) and carbon usage effectiveness (CUE) and the creation of the Data Center Maturity Model (DCMM).
For CIO’s getting ready to update their strategic plans for next year, I would just like to encourage you to incorporate a green data center goal for reducing your PUE. Of course that means you may need to make an initial PUE measurement so you have a baseline using an initial period if you don’t have historical measurements.
Similarly, you may have to settle for using PUE0 initially in your baseline using the UPS instrumentation and work towards the preferred PUE3 method when you can add the needed instrumentation at the actual IT load. But don’t let that stop you from getting started on the road to a really green data center.