A green data center, or sustainable data center, is a service facility which utilizes energy-efficient technologies. They do not contain obsolete systems (such as inactive or underused servers), and take advantage of newer, more efficient technologies. With the exponential growth and usage of the Internet, power consumption in data centers has increased significantly. Due to the resulting environmental impact, increase in public awareness, higher cost of energy and legislative action, increased pressure has been placed on companies to follow a green policy. For these reasons, the creation of sustainable data centers has become essential in an environmental and a business sense. The use of high-performance computing techniques has increased, trading energy consumption for increased performance. Industry estimates suggest that data centers consume three to five percent of the world's global energy. According to an AFCOM State of the Data Center survey, 70 percent of data-center providers indicated that power density per rack has increased significantly since 2013. Managers have been forced to find new ways to power their data centers with renewable energy sources such as hydro, solar, geothermal, and wind. More efficient technologies were developed to decrease data-center power consumption. Several metrics have been developed to measure power efficiency in data centers. Power usage effectiveness (PUE) and carbon usage effectiveness (CUE) are two frequently-used metrics created by the Green Grid (TGG), a global consortium dedicated to advancing energy efficiency in data centers. PUE was invented in 2007, and proposed new guidelines to measure energy use in data centers. This ratio describes how much extra energy a data center needs to maintain IT equipment for every watt delivered to the equipment. The best PUE a data center can have is 1: an ideal situation, with no extra energy use. When PUE was introduced, studies found that the industry-average PUE was between 2.5 and 3. In more recent studies, the average PUE fell to about 1.