Comparing Annualized and Peak PUE

Jessica Ciesla
August 7, 2019

The Green Grid® introduced the Power Usage Effectiveness (PUE) metric in 2007, and it’s currently the de facto standard for measuring a data center’s efficiency. The traditional method of calculating a data center’s PUE is to collect the required data manually and add it to a worksheet at predetermined intervals, whereupon an application calculates the PUE. A more advanced method of obtaining the PUE is to use intelligent rack PDUs that take the measurements and transmit them to a DCIM power monitoring solution that automatically calculates the PUE. Many data center managers are still unclear on what PUE is, how it’s calculated and what it’s used for. Most managers understand that their PUE should be as close to 1 as possible, but they may not know why this measurement is desirable.

What is PUE?

PUE (Power Usage Effectiveness) is the ratio of the total energy consumed by the data center to the energy consumed by the data center’s IT equipment. This ratio tells you much of your data center’s energy is used by its IT equipment and how much is overhead. Two distinct types of PUE exist, including annualized PUE and peak PUE.

Annualized PUE is the ratio of the data center’s annual energy consumption to the annual energy consumption of its IT equipment as indicated by the following equation:

Annualized PUE = (annual energy consumption by cooling + lighting + IT + other electrical power) / (annual energy consumption by IT).

This metric considers variations in daily energy usage, which is primarily due to differences in the outside temperature.

Peak PUE is the ratio of the data center’s annual energy consumption under maximum load to the energy consumption of its IT equipment under maximum load as indicated by the following equation:

Peak PUE = (energy consumption under peak load by cooling + lighting + IT + other electrical power) / (energy consumption under peak load by IT).

This metric is equal to the data center’s PUE on the hottest day for the year for most locations.

Applications

The annualized PUE is typically used to measure a data center’s operational efficiency and helps you to understand what is driving power consumption. Some data center providers alter this metric based on the data center’s total electrical capacity, rather than its actual usage. The peak PUE is typically used to measure the data center’s operational efficiency in a worst-case scenario. It also describes the scale of the data center’s cooling and power infrastructure. In either case, a lower PUE indicates greater efficiency.

Impact

Assume for this example that an aligned data center uses 115,000 megawatt hours of electricity each year. The cost of electricity is $.06 per kilowatt hour, or $60 per megawatt hour. The cost of powering this data center for a year is therefore 115,000 x $60 = $6.9 million per year.

Assume further that this aligned data center has an annualized PUE of 1.15. The same data center would have an annualized PUE of 1.2 when equipped with hyperscale equipment, according to the June 2016 United States Data Center Energy Usage Report by the Lawrence Berkeley National Laboratory. This change would increase the data center’s electrical cost by about four percent, or about $300,000 per year.

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