Efficiency
Data centers receive an efficiency rating based on how their power is utilized. That measurement, Power Usage Effectiveness of PUE is calculated by Total Facility Energy / IT Equipment Energy. This is essentially a ratio of the power to the facility as a whole to the power used solely for computational purposes. A perfect efficiency, which does not exist in reality, would be if all the power was used for IT load and the PUE would be equal to 1.
Currently many options for reducing power consumption and increasing efficiency are coming to the forefront, particularly in the areas of reducing power used for cooling and reducing the power consumption of the servers themselves.