We've all seen the cutting-edge data centers from Apple, Google and Facebook that are ultra-energy-efficient. But what about the rest of the industry, which is made up of companies that may not be building lots of new facilities or that may not have the scale to afford such lavish retrofits? 

A survey of 300 North American corporations from Digital Reality Trust suggests they may be headed in the opposite direction.

Digital Reality Trust reports that the average Power Usage Effectiveness (PUE) rating -- a measurement of how efficiently data centers use energy to operate equipment -- was 2.9 among the companies surveyed. That means those firms are consuming 1.9 extra watts (much of it for cooling) for every watt used to run the servers. That's a very slight increase from a previous survey of North American data centers in 2011, which showed an average PUE of 2.8. And compared to 2007 figures from the U.S. Environmental Protection Agency showing a 1.9 average PUE, these latest numbers look even worse.

Considering the visible efforts in the data center world to cut energy use, this apparent decrease in efficiency is surprising. But the results do more to obscure what's happening in the data center world, rather than shedding light on a specific trend.

"It's important to remember what PUE is: a very good measure of efficiency of the power and cooling infrastructure," said Kathrin Winkler, chief sustainability officer for the IT services firm EMC Corporation. "It is not a measure of the actual efficiency of the IT use."

That's an important distinction when considering server virtualization. As data center operators increasingly virtualize servers and utilize more capacity, they boost the overall efficiency of equipment operation. However, if power requirements stay the same, the PUE does not change, even though the facility is delivering more services per watt.

"If a company has doubled its utilization capacity, then it has effectively doubled the efficiency of the data center without changing PUE at all," said Winkler.

Digital Reality Trust also does not break down the size or age of facilities surveyed. Newer, bigger data centers offer the scale to implement cutting-edge energy-efficiency measures. Older, smaller data centers are often slower to move, either because of cost constraints or a lack of public pressure that companies like Apple and Google might face.

The survey shows that the average power load among the companies surveyed increased from 2.1 megawatts in 2011 to 2.6 megawatts in 2013. This correlates with a .6 kilowatt increase in average power density per rack as companies deploy more powerful servers with better virtualization capabilities. While offering better performance, those servers can also increase onsite cooling demands.

Because the survey only included 300 companies and focuses on PUE as one measure of efficiency, the results don't necessarily show a hard trend. But they may be "directionally correct," said GTM Senior Analyst David Groarke.

"It does raise the point that we probably have different categories of data centers emerging," said Groarke. 

At the top are the ultra-efficient facilities built by the biggest companies; in the middle are the larger and mid-sized companies with a PUE close to what was reported in this survey; and at the bottom are the smaller players with a poor PUE rating that don't have the money to spend on upgrades.

"We have a trend problem" if the latter two categories are growing fastest, said Groarke. Because other global surveys have shown a strong reduction in PUE year-over-year, Groarke said he thinks "the overall trend is a little healthier than this data suggests."

There are some positive indicators in the survey. More than 80 percent of the companies surveyed reported monitoring power use, with two-thirds measuring power at multiple meter locations. In addition, 80 percent of companies say they are using aisle containment to prevent hot and cold air from mixing within a data center. Another 80 percent are using infrastructure management software to better monitor and control equipment in facilities.

However, even with all these changes, the industry-wide efficiency outcome is still unclear.

"Whether or not they are getting more efficient today, there is plenty of work to do," said EMC's Winkler.