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For instance, he wrote that the actual amount of power consumed per server, which has a major impact on total power consumption, needs further research.
He also wrote that servers in 2010 have "higher processing power, more memory, faster network connections, more components, and bigger power supplies" along with "power management and other clever technologies" to cut electricity consumption.
Another unknown is the impact on power consumption from what Koomey termed "comatose" servers, or servers which are powered up but which are no longer delivering computing services. He cited anecdotal evidence that between 10 percent and 30 percent of servers in many data centers fall in this category.
Koomey's assumption of power consumption from data storage devices was also less detailed than that of servers, noting anecdotal evidence that data storage requirements have been growing more rapidly than computing power in data centers while the data density of that storage equipment has also been growing.
"In addition, the total power used by these devices is primarily related to the number of drive spindles, not to the amount of data they hold, so the relationship between total data storage capacity and total energy use is not a simple one," he wrote.
Cloud computing is also beginning to have an impact on data center power consumption given its ability to more efficiently use compute and storage resources than conventional uses, Koomey wrote. However, data on the impact of cloud computing is still sketchy.
"Once such data become available, a more accurate analysis of infrastructure electricity use will be possible, since the cloud computing facilities can then be segregated from the in-house facilities in the calculations," he wrote.
Looking forward, Koomey wrote that IDC forecasts show virtually no growth in the installed server base from 2010 to 2013 as virtualization becomes more prevalent, cutting the need for more physical servers. As a result, he wrote, lower data center power consumption growth can be expected.
Koomey treated Google separately from the rest of the study in that the company is not only the world's largest server user, it also assembles its own servers featuring their own proprietary power saving technology. He cited estimates New York Times reports that Google was running about 1 million volume servers in 2010, up from 25,000 in 2000 and 350,000 in 2005.
Based on a 2008 conversation with Google, Koomey estimated that Google's 2010 total power use was 1.9 billion kWh, or about 0.8 percent of all the world's data center power consumption.
Greenpeace, in a reported it issued in April, wrote that Google is among the top two users of clean, alternative energy, including wind power, solar energy, hydropower, bioenergy, geothermal power, and marine or ocean wave power, in its data centers.