All data-center products claiming to be green are not created equal. That's because metrics to determine how green equipment is usually are vendor-driven and measuring energy efficiency can be a chore for network architects.

"There is currently no widely agreed upon definition in the industry for green," says Scott Scherer, an analyst with In-Stat and co-author of a report, "Green networking equipment: Who leads and who lags." Individual vendors create their own meaning of greenness in measuring how well their equipment stacks up, Scherer says. "Some vendors use 'green' interchangeably with the energy efficiency of their products, while others use more holistic definitions that include corporate social responsibility, manufacturing processes, materials and recycling programs," he says.

Click to see: A chart of PoE power consumption

Such myriad options leads to a range of units of measurement that network executives can sift through when assessing the environmental friendliness of their network components, as well as individual products they consider as they refresh and expand their infrastructure.

One simple measure is how much power a device uses vs. the number of ports it has, so the unit is power per port. This harkens back to the popular measure of cost-effectiveness, price per port. Because port speed and type (copper or fiber) vary, this measure has its shortcomings.

An alternative is the throughput of a device compared with how much power it uses, so the metric would be bits per second/Watt. In practice this would likely be measured in Mbps or Gbps per MWatt.

But device-by-device analysis may not give businesses the big picture they seek, so metrics such as power per user come into play, Sherer says. This takes into account the range of devices that make up the network infrastructure and all the power they consume in order to serve a given worker population.

Because businesses measure productivity based on costs and expenses required to produce products, some look at greenness in the same way, Sherer says. This becomes power per unit of work, a complicated calculation that depends heavily on the type of business a company is in.

Coming up with an objective measure for the unit of work can be challenging, he says. "This is a metric that is user-specific and depends on the type of work that the network actually performs," he says. "These are often very complex and require the equipment for actual testing."