Problems with the Power Usage Efficiency (PUE) metric
This is one of my slides from a recent talk I gave on Cloud Computing in Athens (full talk is at www.slideshare.net/TomRaftery/cloud-computings-green-pote...).
PUE (en.wikipedia.org/wiki/Power_usage_effectiveness) is a widely used metric to measure how efficient data centers are. However it is a poor measure.
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. This is our baseline.
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. Their carbon intensity of supply is 0.8kg CO2/kWh resulting in an IT 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) 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 - the lowest of the three examples and only around 60% of the carbon intensity of the low PUE data center.