Power Usage Efficiency (PUE) is a poor data center metric

Problems with PUE

Power Usage Effectiveness (PUE) is a widely used metric which is supposed to measure how efficient data centers are. It is the unit of data center efficiency regularly quoted by all the industry players (Facebook, Google, Microsoft, etc.).
However, despite it’s widespread usage, it is a very poor measure of data center energy efficiency or of a data center’s Green credentials.

Consider the example above (which I first saw espoused here) – in the first row, a typical data center has a total draw of 2MW of electricity for the entire facility. Of which 1MW goes to the IT equipment (servers, storage and networking equipment). This results in a PUE of 2.0.

If the data center owner then goes on an efficiency drive and reduces the IT equipment energy draw by 0.25MW (by turning off old servers, virtualising, etc.), then the total draw drops to 1.75MW (ignoring any reduced requirement for cooling from the lower IT draw). This causes the PUE to increase to 2.33.

When lower PUE’s are considered better (1.0 is the theoretical max), this is a ludicrous situation.

Then, consider that not alone is PUE a poor indicator of an data center’s energy efficiency, it is also a terrible indicator of how Green a data center is as Romonet’s Liam Newcombe points out.

Problems with PUE

Consider the example above – in the first row, a typical data center with a PUE of 1.5 uses an average energy supplier with a carbon intensity of 0.5kg CO2/kWh resulting in carbon emissions of 0.75kg CO2/kWh for the IT equipment.

Now look at the situation with a data center with a low PUE of 1.2 but sourcing energy from a supplier who burns a lot of coal, for example. Their carbon intensity of supply is 0.8kg CO2/kWh resulting in an IT equipment carbon intensity of 0.96kg CO2/kWh.

On the other hand look at the situation with a data center with a poor PUE of 3.0. If their energy supplier uses a lot of renewables (and/or nuclear) in their generation mix they could easily have a carbon intensity of 0.2kg CO2/kWh or lower. With 0.2 the IT equipment’s carbon emissions are 0.6kg CO2/kWh.

So, the data center with the lowest PUE by a long shot has the highest carbon footprint. While the data center with the ridiculously high PUE of 3.0 has by far the lowest carbon footprint. And that takes no consideration of the water footprint of the data center (nuclear power has an enormous water footprint) or its energy supplier.

The Green Grid is doing its best to address these deficiencies coming up with other useful metrics such as, Carbon Usage Effectiveness (CUE) and Water Usage Effectiveness (WUE).

Now, how to make these the standard measures for all data centers?

The images above are from the slides I used in the recent talk I gave on Cloud Computing’s Green Potential at a Green IT conference in Athens.