I also have one of the wd green 2 tb drives and am interested in building a system that could take several (say 4 max) of the same or similar drives when I want to upgrade later. In the specs they quote 6 W for read/write, but will the drives ever be drawing significantly more than that, say on startup or spinning up from idle? Also, is much of that power coming over the 5 V line at all?
We don't have devices that can measure and log transient power accurately enough to check startup power for HDDs, but every manufacturer cites much higher power demand for this. WD says 1.55A peak on the 12V line for 2TB Greens. That's 18.6W just on the 12V line. There's some power drawn on the 5V line, but it is much lower power than the 12V line. For example, on the WD Green 3TB, max measured current at seek on the 5V line was 0.45A, which translates to 1.8W -- compare this to 0.44A on the 12V line = 5.28W. In any case, 25W for max peak at spinup is not an unusual spec for drives. I would certainly make sure the PSU was rated to provide at least 20W per drive for a second or 2 anyway. The pertinent spec is peak power capability -- most PSUs can do higher than rated power for a brief period.