Today I attended The Green Grid meeting in Paris at the Schneider Electric headquarters.
The Green Grid has made itself a name by being able to have three continents (US, Europe, Asia) agree on a clear definition on how to calculate the PUE (Power Usage Efficiency) metric. The PUE value inside a data center is the power ratio of the total power used by the whole facility divided by the power consumed by IT equipment. A PUE value of 1.3 means e.g. that 30% of the total power used is for cooling, power loss and other facility sources on top of the power used for IT (1.0). So the smaller the value, the more efficient the data center. Of course this value depends on when it is being measured. During a cold winter day it will be better than during a hot summer day assuming free cooling is being used. The Green Grid wants data centers to measure the value over a period of 12 months, which makes lots of sense.
Now PUE might not necessarily be the best value to use, although it is great as mentioned that a metric has been agreed on internationally. There are the following shortcomings:
- PUE does not include information on availability, a more redundant data center would have naturally a higher PUE than a very low or not redundant data center
- total utilization of a data center is also not reflected, a new big data center with few clients can have the most innovative green measures in place but still achieve a bad PUE
- it is also very possible that measures reducing IT load such as consolidation of servers enabled with virtualization increase the PUE
- while electrical energy is important to look at also water used, CO2 production, reuse of waste energy and other resource efficiencies are not reflected
- an older data center will most likely have PUE values that will be higher than a new data center with latest innovative cooling systems used