The Internet is sometimes described as a cloud -- a big, borderless area of computing power. But this power has to come from somewhere.

Data centers are buildings full of servers. These powerful computers store, process and move information. They make the Web work.

"Server farms" contain thousands of machines -- millions in all at data centers worldwide. And there is strong demand for more as a major change takes place in computing.

Tasks traditionally done with software stored on a personal computer are now increasingly offered as services on the Internet.

Google Apps, for example, combines e-mail, word processing and other applications. Groups can work together online without the need for special software.

Other companies, including Salesforce.com and Microsoft, also offer software as a service.

Rich Miller is editor of Data Center Knowledge. He says the number of data centers has jumped in the last three or four years. But all that computing power takes another kind of power which also has to come from somewhere.

Data centers need energy not only to operate their servers but to cool and protect their systems.

The Environmental Protection Agency says the United States has about ten thousand data centers. The E.P.A. estimates that they use almost two percent of the country's electricity. In two thousand six, the most recent year reported, power costs reached four and a half billion dollars.

The amount and cost of energy use by data centers is the subject of growing concern and criticism. Some data centers use only about twenty to thirty percent of their computing power.

Using wind power or other renewable energy for data centers could help the situation. Another answer is to make servers more energy-efficient. The E.P.A. has recently set rules for measuring and rating the energy use of servers.

Still another solution may come from virtualization. This lets one server work like several. Usually, a server runs a single operating system, like Linux or Windows. Virtualization lets both run at the same time.

Rich Miller at datacenterknowledge.com says the need for data centers will only increase as the Internet becomes more central to everyday life.

As applications are becoming more complex, so are data centers. Many companies are having trouble finding enough skilled people to manage them.

And that's the VOA Special English Economics Report, written by Mario Ritter. I'm Steve Ember.