A new study says that 30% of all physical servers in data centers are comatose, or are using energy but delivering no useful information. What's remarkable is that that percentage hasn't changed since 2008, when a separate study showed the same thing.

The latest research was reported in a paper by Jonathan Koomey, a research fellow at Stanford University, who has done data center energy research for the U.S. Environmental Protection Agency, and Jon Taylor, a partner at the Athensis Group, a consulting firm.

The researchers used data collected by TSO Logic, an energy efficiency software vendor, from nearly 4,000 physical servers in customer data centers. A server is considered comatose if it hasn't done anything for at least six months.

The high number of such servers "is a massive indictment of how data centers are managed and operated," said Koomey. "It's not a technical issue as much as a management issue."

This work adds to the findings of two other groups that have looked at the problem.

In 2008, McKinsey & Co. released its eye-opening finding that up to 30% of servers in data centers are "functionally dead." The McKinsey finding was backed-up by the Uptime Institute in 2012, which also arrived at the 30% figure based on data collected from its customers.

In 2014, Uptime put the figure at 20%, but that lower figure is attributed to data collection variability and not to a dramatic improvement from 2012. Uptime puts the number of comatose servers at 20% to 30%, said Matt Stansberry, Uptime's director of content.

Koomey, however, cautions that while the McKinsey and Uptime offer importance pieces of information, more work is needed to confirm or refute those numbers.

Koomey said the plan is to continue to work with TSO Logic and produce ongoing reports with larger data sets. The TSO software gathers utilization data from the servers, but also looks at how much data is going in and out of servers, he said.

IDC estimated the number of physical servers worldwide last year at 41.4 million; that figure is expected to grow to 42.8 million by the end of this year.

A study last year by the Natural Resources Defense Council (NRDC), with the help of major vendors, estimated that in the U.S. alone data centers used 91 billion kilowatt-hours of electrical energy in 2013. That use is expected to increase 53% by 2020. It estimated that electrical usage could be reduced by 40% by getting rid of zombie servers and improving energy efficiency. That figure represents only half of the technically possible reduction in energy use.

The NRDC believes that the problem is concentrated in smaller data centers, and not in the large cloud centers.

"The problem is not getting better," said Pierre Delforge, director of the NRDC's high-tech sector on energy efficiency, about the latest findings.