Saving energy is always a plus and the reduced electricity use here at $.42/KWh or so here in California is highly attractive, but energy usage does not exist in a vacuum. I am speaking of the need for external expansion with everything. Is automobile mileage calculated with all seats and interior ripped out to save weight?
To use a 2013 Mac Pro as a workhorse like the older “less efficient” 2010-2012 models, drives have to be connected, compatibility and expansion boxes connected, etc. These devices have their own power draw, and chances are high that they are substantially less efficient for the same functionality.
For example, it is not efficient to have 3/4/5 power bricks powering a gaggle of devices. It is efficient to have one very high-efficiency power supply making power for the computer, various internal drives, etc.
Moreover, even assuming that the external devices themselves are as power efficient as the Mac Pro (highly unlikely), power bricks can be “vampire” devices that continue to suck power even when the device is not in use.
The only intellectually defensible way to assess power usage is to add up all the power costs for an equivalent configuration of the prior model vs new model, e.g., with all the SSDs and drives and so on needed.
Of course, there is variation in setup. Presumably a few users can get by with only the internal SSD in the 2013 Mac Pro, and can use storage on a NAS and have their IT staff do backups. Or something like that. But most Mac Pro users will need Stuff attached.
In my 2010 Mac Pro, I have 2 internal hard drives, 2 PCIe SSDs, 3 SATA SSDs, and an eSATA card. These are all powered by the 2010 Mac Pro tower’s power supply. I use them all—heavily.
In the 2013 Mac Pro, all of my devices now have to be relocated into external enclosures. The power draw will rise for those devices (all those enclosures and fans and power bricks), canceling out some of the savings of the 2013 model. I expect lower total power draw (I hope).
I have a WattsUp power meter, so I can make some estimation of the differences, but my 2010 Mac Pro is also a 12 core with 80GB and so comparing that to my planned 6-core with 64GB itself isn’t entirely fair. Still, the idle power usage is of some interest, more so if the performance is similar.