Power usage effectiveness (PUE) is a metric developed by The Green Grid that measures data center efficiency. It’s a ratio of the total power consumed by a data center to the energy delivered to the servers. The ideal PUE is 1.0, meaning 100% of the data center energy powers the IT equipment. Best practice PUE for the IT sector is 1.5.
Water Usage Effectiveness (WUE), developed by The Green Grid, is a ratio of data center water use to the energy delivered to the servers. Facebook cares about using water efficiently and pioneered reporting WUE in Q4 2012. Since WUE is a relatively new measure, there are currently no industry standards or baselines.
Indoor humidity in our data centers is controlled through an evaporative cooling system. Typically, when it’s hot, the humidity is low. This allows evaporative cooling to be effective, and it increases our water use. When humidity is low, the mechanical systems consume water to add humidity, which also increases WUE.
Servers need to be kept cool in order to function properly. Our servers function in a wider range of temperatures than standard models, which is part of why we operate so efficiently. Hot server air is cooled via outside air economization and, if need be, through evaporative cooling. Evaporative cooling yields higher instantaneous PUE and WUE numbers.
Annualized Numbers — The chart above shows real-time PUE, WUE, temperature and humidity for Facebook’s Forest City data center. The numbers to the right are the Forest City data center PUE and WUE as of the end of December 2013.
We are sharing dashboards that show real-time PUE (power usage effectiveness) and WUE (water usage effectiveness) at our data centers in Oregon and North Carolina. We’d previously been sharing PUE and WUE quarterly and are now increasing the frequency of that sharing through these dashboards.
We work hard to maintain extremely efficient data centers, and we use data including PUE and WUE to track the performance of these facilities. We’ve also started to share data on our energy use and carbon emissions; you can find that information here.