Comment by meatmanek

Dedicated NAS hardware is often pretty efficient. The Synology DS224+ (a 2-bay model) for example claims to idle at 4.41W when the drives are allowed to hibernate. That's within spitting distance of a Raspberry Pi, and a lot better than most people will be able to achieve with repurposed desktop/server hardware. Lots of desktops will idle at tens of watts.

If you keep your spinning rust drives spun up all the time, it's another 5ish watts per drive.