Energy standard update includes alternate path for power usage effectiveness

ANSI/ASHRAE/IES Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings is the industry standard regarding energy efficiency and is referenced in many energy and building codes. It was first published in 1975. It has been under continuous maintenance and refinement by the ASHRAE standards committee and, since 2001, an update has been published in the fall of every third year.

The latest edition, published in 2013, contains new sections to provide for an alternate method or path to compliance with the energy standard. This marks the first time an alternate path has been provided based on power usage effectiveness, or PUE. This metric was developed by the data center industry sector. Power usage effectiveness is the total energy used by a computer room or data center, divided by the energy used by the IT equipment.

The latest ASHRAE standard sets a maximum value for PUE, based on the data center climate zone location. This defines a “ceiling” for how much additional energy can be used by air conditioning systems, lighting systems, et cetera, ranging from 1.61 (IT equipment energy plus 61 percent) in the hottest climates, down to 1.30 (IT equipment energy plus 30 percent) in the coolest climates. For this compliance path, the architect and engineer must design the data center building envelope, AC and lighting systems to stay below the energy use ceiling to comply with the energy code.

The alternate path was developed for computer room systems with the assistance of ASHRAE’s data center Technical Committee 9.9. This committee has been very active in updating design standards to maximize data center energy efficiency. Particularly, TC 9.9 has shown that data system air conditioning systems often are over-designed because of sizing air conditioning equipment for unnecessarily low space temperatures. In past years, it was customary to design computer room air conditioning systems to maintain 68-72 F, when modern IT equipment often can run in a space temperature of 78-80 F with no adverse effect on performance or reliability.

This new alternate compliance path is important, because it opens the door to consider a similar approach for energy using systems that are not easily covered by other compliance paths.

For more information, contact Jonathan Noordzy at or Robert Knoedler at