This is not just another article about power usage effectiveness (PUE) and how to measure or improve it. Just when you thought you were a responsible, efficiency conscious data center manager and were comfortable with your data center’s PUE, ASHRAE’s proposed new 90.4 standard may be about to move your cheese. Are you prepared to prove your power usage effectiveness (PUE)of your facility to the local building inspector?

The Green Grid (TGG) first introduced the PUE metric in 2007 that started the data center industry thinking about the energy efficiency of the physical facility. Its purpose was to help data center operators to baseline and improve their own facility. PUE has been criticized by some, since it only covers the energy efficiency of the facility (not the IT systems), however, that was its clearly stated purpose. Since then, PUE has been updated over the years and is now the process of becoming an ISO standard. Nonetheless, it is its underlying simplicity that allowed managers to easily calculate (or guess) a facility’s PUE that has been key to its adaption as a de facto industry metric.

While PUE was never meant to compare one facility to another, it still resulted in PUE bragging rights. Google first claimed a PUE of 1.21 in 2008 which at the time was seen as ground breaking; since the average traditional enterprise facility was operated at a PUE of 2 (many sites were higher). This first became apparent when the EPA undertook its first survey in 2008 of 120 data centers that volunteered to participate of which ultimately formed the basis for the Energy Star for Data Center’s certification program award (which became effective in 2010). The Energy Star program continues to be a voluntary program, however, and after more than six years only has 72 facilities listed (

Claims of low PUE numbers have also become a status tool for many data centers by corporate organizations seeking to demonstrate their sustainability achievements. There have also been marketing claims by some colocation or other services providers who may have cited PUE numbers that were not really accurate (in some cases they did not include all the power related to the facility’s operation). In some instances, marketing departments deliberately used calculations based on a single power “snapshot” taken on a cold winter night (when the mechanical cooling systems were measured at their lowest kW draw), which of course resulted in a low PUE. This snapshot power measurement was one of the loopholes of the original version of PUE metric. Subsequently in 2011, TGG updated the PUE metric to version 2, which specified that it should be based on annualized energy usage (

One of the other issues with PUE is that it does not have any geographic adjustment factor. This means that since cooling system energy typically represents a significant percentage of the facility energy usage, identically constructed data centers would each have a different PUE if one were located in Miami while the other was in Maine. But PUE was, and still, provides a simple uniform number which is easy to understand and calculate regardless of location.

Nonetheless, while data centers may be different than an office building, like any other building, they still need to comply with local and state building codes for safety, and more recently for energy efficiency. In many areas of the U.S. the ASHRAE 90.1 standard is referenced and incorporated as part of local building codes. Data centers were previously more or less exempted in the 90.1 standard, however as of 2010 data centers were included. This raised some concerns in the data center community because the requirements were prescriptive in nature rather than goal oriented. Even Google voiced its concerns (

This past year, ASHRAE seems to have tried to address these issues with the creation of the proposed “90.4 Energy Standard for Data Centers.” However while its intention may have been to address the highly prescriptive concerns contained in 90.1, the proposed 90.4P standard also has been criticized by the data center industry since it mandates compliance of specific annualized PUE for data centers.

The “Second Draft for Public Review (September 2015)” of the proposed ASHRAE 90.4p standard explicitly lists maximum PUE ranges from 1.30 to 1.61, each specifically related to a location listed in each of 17 ASHRAE climate zones. The 66-page, highly detailed draft also covers specific compliance requirements for minimum energy efficiency for the mechanical cooling systems, again specifically listed for each climate zones, while 90.4 attempts to address geographic effects on cooling efficiency by correlating PUE to local conditions. However, it should still be up to the owners as to where to build the facility and make their own decisions as to the total cost of ownership (TCO), not just based on PUE, but other economic factors such as the cost of energy or tax incentives, as well as their own strategic business objectives.

Moreover, there also seems to have been an internal “disconnect” between the pending 90.4 standard and ASHRAE’s Technical Committee 9.9 (TC9.9), which is the source of the “Thermal Guidelines for Data Processing Equipment.” In fact, it has caused Don Beaty, who is well respected ASHRAE Fellow and the co-founder and the original chair of TC9.9, to publically voice his concerns declaring; “The industry has got to rally. You shouldn’t use formal standards in a rapidly growing, fast-moving industry.” The fact that Beaty made this comment publicly shows the serious nature of the potential impact to new data centers.

Ironically while TC-9.9 has been the source of “Thermal Guidelines for Data Centers,” which is widely considered an industry bible by the majority of data center operators, it is not legally recognized by any governmental agencies responsible for overseeing and enforcing building codes related to the design and construction of buildings.

While focused on cooling performance, the 90.4 standard also details and limits the total maximum electrical losses though the entire power chain, from the utility handoff, through the UPS and distribution system, and ending at the cabinet power strips feeding the IT equipment. Moreover, this all described by the very strictly prescribed “Electrical Efficiency Compliance Paths” along with calculations and specific limits on energy losses for varying levels of redundancy — N, N+1, 2N, and 2(N+1) — as well as the UPS operational design load efficiency specifications at 50% and 100%.

However while its intention may have been to address the highly prescriptive concerns contained in 90.1, in point of fact 90.4 is not a not substitute for 90.1. It references 90.1 as part of the compliance path requirements in the “Energy Prescriptive Compliance – Checklist” section of the proposed 90.4 standard.



I have long been a proponent of data center energy efficiency and have written and spoken about energy efficiency and PUE many times, even before the introduction of the PUE metric by TGG. Unfortunately, it is true that prior to the PUE metric many organizations owned their own data centers and were not very aware of their facility energy efficiency. Since then, new data centers have become much more focused on energy efficiency. In addition, many existing data centers have been taking steps to improve their efficiency.

In addition, the more recent shift toward colocation and cloud service providers have directly or indirectly made energy efficiency a competitive mandate and part of the justification for lowering enterprise computing TCO. However, unlike other worthwhile goals of organizational sustainability and environmental stewardship, wherein some of those recommendations may add to operating costs, optimizing energy efficiency also makes good economic business sense. Data centers are a necessary part of business operations and energy is one the largest aspects of its operating costs. Therefore, it is inherently in their own self-interest for enterprise organizations, as well as colocation, hosting, and cloud services providers, to optimize their energy efficiency.

While the proposed 90.4 standard may be a well-intentioned effort to improve efficiency, it can become a nightmare if it becomes adopted as a legal requirement to be interpreted and enforced by a wide variety of state and local building departments, some of whom may be unfamiliar with the constantly evolving data center designs and newer technologies.

Perhaps now would be a good time to consider reviewing 90.4 before it is officially released. You might also start thinking of what you would say if asked: Please provide your facility’s projected and operating PUE and compliance to ASHRAE 90.1.and 90.4 standards. So in the interim, keep your eye on your PUE and stay tuned as this unfolds.