I wanted to address all of the comments that state that the need for external peripherals undoes any energy savings in the Mac Pro. That's not necessarily true. Yes, there will be some Pro stations that are individually attached to a large external storage array. For individuals running their own Mac Pro, power consumption may creep up back to the levels of the old box.
But that's not the only use case. A large number of Mac Pros that go out are used as nodes in a render farm, and that's where these energy savings are going to add up. Those setups are already using NAS anyway, so the price for external storage is already being incurred. But using less than 50% of the power per compute node compared to the old boxes -- the savings will quickly add up there.
And even with individual workstations, there could still very well be power saving compared to the old big box. Let's say you hook up a 4 TB hard drive. It's going to draw less than 10 W at idle, and if you buy one for energy efficiency, less than 5W. Considering the new Mac Pro runs 90W lower than the previous generation at idle, you're still going to be looking at considerable savings.
It's not so much about the power savings, its about the carbon footprint and material consumption.
Making an external thunderbolt case, or PCI chassis, with cables and it's own power supply consumes more resources and produces more carbon than an internal drive using an existing computer power supply.
In this regard, the nMP is likely going to be less environmentally friendly than the old MP - particularly in regard to plastics (both for external peripherals, and their packaging).