The three main considerations for power and cooling are; capacity, density and efficiency. Capacity is typically used in planning the power you need to provision. Density for cooling supply and airflow. My boss Doug "Dawg" Gourlay likes to call this Air Supply after his favorite band... Then there is efficiency. The later metric can be applied to a box, system, architecture and of course travel.
Box level efficiency is almost purely dependent on the products power supplies as the power distribution within the chassis is Direct Current (DC) and rarely gets re-converted. Almost all IT equipment uses an Alternating Current (AC) Switched Mode Power Supply (SMPS) these days. These power supplies are typically sourced from Asia and have different levels of quality that can be specified. The quality typically relates to how efficient said supply is. For years Cisco has been paying a premium for highly efficient power supplies, particularly in the data center. Prior to roughly 2002 AC power supplies had efficiencies in the range of 70% efficiency at optimum load. Why did their efficiency jump up so high? This was a carry over from the .com boom where space efficiency was the big concern - we had plenty of power but not enough space. Therefore the likes of Exodus and GlobalSwitch pushed equipment suppliers to make smaller boxes. So how do you do that? You make more efficient components. So highly efficient power supplies are readily available on the open market today.
The best way to compare one box to another box is to first start by looking at the power supplies. This is purely predicated on how the supply is loaded. We at Cisco have power supplies that are ~90% efficient when loaded at 70% or higher. As an example, if I have a power supply that requires 1000 Watts I want to make sure that power supply is on average drawing 700 Watts. If I do this I am in the "efficiency sweet-spot". The next level of box and ultimately systems comparison is much more complex. Power per port is a good starting point but must be blended with a use case. This involves a back and forth if you will between vendors and customers. First, the user must clearly define a use case, then the vendor can show how the feature sets in that product or solution address the use case. So in essence its Power Per Port to do "what" or what we typically refer to as Power Per Work Unit Performed.
Given the complexity of IT operations this is still very much an emerging science. The broad scope of interoperability, scale up, scale out, virtualization, etc. of systems level comparison, this is not a simple task. Given the fact there is only a small difference at the box level, we've been laser focused on driving efficiency within the architecture and through improved asset utilization using things like storage virtualization. In case you have been wondering this is why we only engage on efficiency analysis at the product level with customers who ask us to and when the comparison has to do with operative efficiency using sound metrics. We are not adding to the "Green-wash" by using misplaced metrics and over-simplifying what can be a very complex comparison.
So the moral to the story is while capacity, density and efficiency are all intra-related they are not the same. Don't confuse capacity requirements of a single box with that boxes operative efficiency or systems efficiency. A good way to think of it is the automobile example which Omar Sultan described in earlier postings. My spin on his example is a Prius versus a Big Rig. Which is more efficient?
Please answer back if you know the answer. I would also be very interested if there are any facilities professionals reading and would like to comment on efficiency analysis.
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