For end-users and the industry to fully get behind and realize the full potential of green initiatives, standards are imperative - especially if the standards recognize both dimensions available for improvement: reducing power consumption and increasing power efficiency.
Reducing power consumption is just what it says - reducing power while doing the same amount of work. Ultimately it's measured and compared in watts, although there are advantages in normalizing some kind of work to watts (GB/W, IOPs/W). SNIA's Idle Test Specification employs a GB/W metric for measurement and comparison purposes.
Increasing power efficiency refers to doing more with the amount of power that's available - some power reduction may also be realized, but the focus is on doing more work with the power you have (as opposed to doing the same amount of work with less power). The comparative metric for power efficiency will be whatever work you're concerned about relative to the amount of power consumed performing that work.
Which dimension is most important to you is highly dependent on the application and operating environment. High-performance, primary-storage systems used on mission-critical systems need to get the most work done with the amount of power available, whereas archive- or secondary-storage systems should be designed to minimize the overall power used.
At the end of the day, it is ultimately about economics. Can you save money or get more done within your existing budget by deploying "green" storage?
Action Item: While storage isn't the primary driver of power consumption within a datacenter, it is a substantial-enough power-consumer to warrant attention. Datacenter managers should be keenly aware of the type of application and operating environment running on the storage system and drive appropriate and relevant power reduction and power efficiency requirements back to their storage providers. Be specific on which dimension(s) are important to you and demand continuous improvement.