The key element of EPA ENERGY STAR® for data center buildings is the mapping by the Portfolio Manager of industry-standard metric PUE to ENERGY STAR's 1-100 building rating. Exactly how does this work?

Vertatique and RIA partnered to run a series of tests with Portfolio Manager. We used total and IT energy loads such that each hypothetical data center building had the same PUE and energy intensity. Even so, the ratings varied among the buildings.

It's worth starting the discussion with a quick review of the difference between the two metrics.
1. With PUE, the lower the rating the better. With ENERGY STAR, the higher the better.
2. The best PUE score would be 1.00 (100% of total energy goes to power IT gear). The best ENERGY STAR building rating would be 100 (PUE is equal to the best PUE predicted by ENERGY STAR's algorithm for this building type).
3. PUE is an absolute rating calculated independently of facility size (in either area or energy use) and can be applied to both IT buildings and IT spaces. ENERGY STAR is a measure of an IT building's actual PUE against a PUE predicted by a size-related (in IT energy use) algorithm derived from a data center building reference set and does not apply to spaces[1].
Let's see how this plays out in out tests.

We entered into the Portfolio Manager data for three hypothetical data centers. The data centers scaled in area by an order of magnitude, but kept a constant energy intensity per square foot and a constant total/IT energy ratio across the series.

Update 2010.07.14
NetApp's RTP Data Center (NC, USA), an in-house R&D facility, is the first data center to earn an ENERGY STAR rating. "To earn the ENERGY STAR, NetApp implemented the following features in the RTP data center:
• 74°F average supply air temperature: Using a higher temperature threshold on supply air (74°F instead of 55° to 60°F) allows NetApp to dramatically reduce cooling costs.
• Airside economizer: The data center is cooled by using just outside air ("cooling) 67% of the time during the year.
• Pressure-controlled room: Modulating fans, based on NetApp's proprietary technology, supply pressure-controlled rooms and regulate the volume of air to avoid oversupplying air and wasting energy.
• Cold aisle containment: The cold room separates the cold and hot air streams to protect supply air temperatures from being affected by hot air returning from the racks.
• Overhead air distribution: Instead of pumping cold air up through the floors (raised floors), overhead air distribution takes advantage of cold/hot air buoyancy and eliminates ductwork, reducing the energy needed for fans."

Update 2010.08.17
Consonus' South Data Center (UT, USA) "is the first commercial data center in the nation to achieve an ENERGY STAR rating ..Some of the energy-efficient components and green initiatives used at the Consonus South Data Center include:
• Cold Aisle Containment
• CFD Modeling and Analysis
• Ducted Return Systems
• High Efficiency Floor Tiles"

BNY Mellon's Northpointe Data Center (PA, USA) is in-house facility. "The team focused on utility improvements and successfully implemented a variety of solutions so that the center's critical systems such as the power and cooling supply remain in synch with the workload."

Update 2011.02.08
An email from the ENERGY STAR Commercial & Industrial Program announced

New Optional Inputs for the Data Center Space Type
Two new optional IT Energy meter types will be available in Portfolio Manager. The meter types are optional and therefore will not affect the ENERGY STAR rating calculation. The two new types are:
· Power Distribution Unit (PDU) Output Energy
· IT Equipment Input Energy (meters on each piece of equipment)

The email implies these will be available in the Portfolio Manager on 21 March.

[1] We understand that The ENERGY STAR data center rating system applies only to data center "buildings" and not to data center "spaces". Portfolio Manager appears to roll a data center space's energy consumption into the total building's consumption and calculates an ENERGY STAR rating based on the algorithm for that building type. Also, while ENERGY STAR refers exclusively to 'data center" and 'IT' in its publications, it has confirmed to us that other ICT buildings with independent cooling and energy sub-metering can be rated.