The Green Grid took the wraps off two new metrics this week, carbon usage effectiveness (CUE) and water usage effectiveness (WUE), that will help data center operators paint a clearer picture of just how sustainable their facilities truly are. And just in time, too. As Facebook has discovered, a green data center is judged by more than just its PUE (power usage effectiveness) rating. Regardless of how efficiently the facility utilizes electricity, it also has an impact on air quality — depending on the local utility’s energy sourcing and efficiency, of course — and regional water sources.

According to the industry group:

CUE will help managers determine the amount of greenhouse gas emissions generated in delivering work from the IT gear in a data center facility. Similarly, WUE will help managers determine the amount of water used by the facility, and the amount used to deliver work from IT operations.

Details on calculating CUE are available via a whitepaper on the group’s website. Essentially, calculating CUE for facilities that get all their power from the grid and generate no local CO2 is as simple as plugging in values for the formula below.

Here’s how it works:

“Total Data Center Energy” is the same value as the numerator of the PUE metric. The numerator in this CUE metric is the total carbon emissions caused by the use of the energy in the PUE metric. The units of the CUE metric are kilograms of carbon dioxide (kgCO2eq) per kilowatt-hour (kWh).

The Green Grid also provides alternative ways of arriving at the value of CUE depending on a site’s energy sourcing mix.

Methods of calculating WUE will be released in early 2011. Expect CUE and WUE to contribute to the green data center one-upsmanship that’s taken root among top-tier data center operators. Also expect these metrics to be invoked if they help offset less-than-stellar PUE ratings. Regardless CUE and WUE help eliminate some of the blind spots that emerge from relying solely on PUE as a measure of a data center’s sustainability, and that’s a big step forward.