Data-center power glossary

CRAC, DCiE, REC — and other cooling and powering terms that’ll get facilities managers to take you seriously

You need to learn a whole new vocabulary when you start talking with your company's facilities team about lowering data-center energy use.

If you thought IT acronyms were hard to remember, wait until you sit down with your facilities team to discuss your data center's electric bill. You need to learn a whole new vocabulary when you start talking about lowering the building's energy use.

Here's a crib sheet of a dozen of the most commonly used energy terms and acronyms so you can learn the jargon for going green.

1. AC/DC

Yes, this is the name of Australia's greatest rock band, but it's also a key trend in data-center design. AC stands for alternating current, and DC stands for direct current. Leading-edge data-center designers are looking at power supplies based on DC power -- rather than today's AC power -- because DC power promises to be more energy efficient.

2. Carbon footprint

No relation to Sasquatch, although to corporate executives it can be an equally large and scary beast. A company's carbon footprint is the amount of CO2 emissions its operations produce. In setting goals to reduce their carbon footprint, many companies target their data centers because they consume 25% or more of the electric bill.

3. CFD

It sounds like the acronym for the Chicago Fire Department, but this version stands for computational fluid dynamics. CFD high-performance-computing modeling has been used for a long time in the design of airplanes and weapon systems. Now it's being applied to air flow in data centers for optimal air-conditioning design.

4. Chiller

This isn't what you drink at the beach on a hot day. Rather, it's a machine that uses chilled water to cool and dehumidify air in a data center. Of all the components of a data center's air conditioning system, this is the one that consumes the most amount of electricity -- as much as 33% of a data center's power.

5. Close-coupled cooling

This sounds like a technique that would come in handy on Valentine's Day. In fact, it's a type of data-center air-conditioning system that brings the cooling source as close as possible to the high-density computing systems that generate the most heat. Instead of cooling down the entire room, close-coupled cooling systems located in a rack cool the hot air generated by the servers in just that rack.


This is not what you sometimes see when a plumber bends over, although it's pronounced the same way. We're talking about a computer-room air-conditioning system. CRAC units monitor a data center's temperature, humidity and air flow. They consume around 10% of a data center's power.

7. DCiE

This acronym has nothing to do with the nation's capital, although its pronunciation is similar. DCiE is the Data Center Infrastructure Efficiency metric (also called DCE for Data Center Efficiency). DCiE is one of two reciprocal metrics embraced by The Green Grid industry consortium; the other is Power Usage Effectiveness (PUE, below).(See "Two ways to measure power consumption.") DCiE shows the power used by a data center's IT equipment as a percentage of the total power going into the data center. A DCiE of 50% means that 50% of the total power used by a data center goes to the IT equipment, and the other 50% goes to power and cooling overhead. The larger the DCiE, the better.

8. kWh

Electric power is sold in units called kilowatt hours, 1 kWh is the amount of energy delivered in one hour at a power level of 1000 watts. This abbreviation for "kilowatt hour" is mostly used in writing rather than conversation.

9. PDU

The acronym PDU stands for power distribution unit, a device that distributes electric power. PDUs function as power strips for a data center and consume around 5% of the power in a typical center.

10. PUE

Not pronounced like the reaction to a bad odor, but one letter at a time. Power Usage Effectiveness is one of two reciprocal metrics embraced by The Green Grid industry consortium; the other is Data Center Infrastructure Efficiency (DCiE, above). PUE is the ratio of the total power going into a data center to the power used by the center's IT equipment. For example, a PUE of 2 means that half of the power used by the data center is going to the IT equipment and the other half is going to the center's power and cooling infrastructure. Experts recommend a PUE of less than 2. The closer a PUE is to 1, the better.

11. RECs

Pronounced like the short version of the word recreation, this acronym means renewable energy certificates or renewable energy credits. RECs are tradable commodities that show that 1 megawatt-hour of electricity was purchased from a renewable source, such as solar, wind, biomass or geothermal. An increasing number of companies are buying RECs to offset the amount of electricity generated from fossil fuels that their data centers consume.

12. UPS

We're not talking about the boys in brown, although the acronym is pronounced the same way. We're talking about uninterruptible power supply, which provides battery backup if a data center's power fails. It's essential that UPS equipment be energy efficient, because it consumes as much as 18% of the power in a typical data center.

< Return to main NDC page: Power: What you don’t know will cost you >

Learn more about this topic

Energy-efficiency self-assessment tool


Two ways to measure power consumption


Where to turn for advice about power


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