ASHRAE Journal - June 2021 - 37

TECHNICAL FEATURE
* Colocation providers use PUE to compare themselves
to one another for marketing purposes as well as a
basis of the contracts they sign with their customers.
* Enterprise data centers use PUE to drive energy effi
ciency in their business, for energy rebates and incentive
programs and for public relations purposes.
Regardless of how the metric is used, for the industry
to truly benchmark and compare facilities there must
be consistency across calculations and the method with
which they are approached.
PUE: A Comprehensive Examination of the Metric
The book's introduction states, " As there are various
ways to calculate PUE, stakeholders in the industry have
expressed concerns around the consistency and repeatability
of publicly reported measurements. " This indicates
that the calculation of PUE is one challenge and
that its measurement is another. In theory (calculation)
and in practice (measurement)* the two efforts should
be approached as similarly as possible. Despite the
book's being centered around the measurement aspect,
most of its information can be directly applied to the
respective calculation for peak or annual PUE (shown in
the next section).
Chapter 3 states, " To further enable equitable comparison
of PUE results among data centers, additional
attributes such as age, geographic location, capacity
loading, size of facility, infrastructure resiliency and
the like should be taken into consideration. " An interpretation
of this could be that the age of a facility needs
to be accounted for to level-set comparisons between
facilities of signifi cant age and newer facilities.
The purpose of the metric is to indicate objective,
demonstrable effi ciency through concrete numbers, calculations
and measurements, but the book's statement
indicating that the age of a facility should be factored in
to provide " equitable " comparisons introduces a largely
subjective factor that could have signifi cant effects on
projections and measurements.
If the age of the facility has negative impact on its effi -
ciency, this fact should be accurately represented within
the PUE, much like it is understood that a vehicle that is
decades old will typically not have comparable effi ciency
to its brand-new counterpart. Objective application
of the metric to aging facilities would serve as a tool in
driving energy effi ciency improvements and potentially
incentivize owners through grants and programs.
Consideration exists within the book regarding dedicated
data center buildings versus data centers within
mixed-use buildings. Unless a building is strictly a
data center, its support spaces, circulation spaces and
human spaces associated with data center operations
and security teams should be considered mixed-use.
To standardize calculations, it is necessary that PUE be
limited to processes directly related to the operation of
the IT equipment. Furthermore, any power consumption
associated with activity not directly supporting the
IT infrastructure should be exempt from consideration
when calculating or measuring PUE.
The location at which the measurement for total facility
power is taken, or considered (if you're doing peak or
annual calculations-shown in the next section), should
be dictated by whether the facility is a dedicated data
center or mixed-use facility.
For mixed-use facilities, the total facility power
measurements or consideration should take place
at the location at which the critical and noncritical
power sources diverge. This would normally mean just
upstream of the transformer(s) associated with critical
block(s) and critical mechanical block(s) in situations
in which the critical mechanical equipment is fed from
dedicated mechanical blocks. (A critical block is the
electrical infrastructure serving computer processing
or computer processing and associated infrastructure
when on a shared electrical system; a critical mechanical
block is electrical infrastructure serving the mechanical
systems dedicated to conditioning the critical power
waste heat when served by a separate electrical infrastructure
than the computer processing.)
The book offers a very detailed level of information
on what the measurement is, where measurements are
to be taken and even provides matrices with guidance
of different levels of granularity† within the methodology
of calculating and measuring this metric (both in
accuracy and defi ned intervals). This component of the
book applies directly to the measurement aspect of PUE,
*For the purposes of this discussion, the measurement of PUE will refer to the measurement of the data and the resulting calculation.
†It is the author's recommendation to take the most accurate approach possible when measuring or calculating PUE. This includes all
electrical losses upstream of the IT equipment. In practice, however, limitations often exist to what a building owner can measure. In
cases such as these, the recommendation is to match the calculation to the intent of the design.
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