Not sure if that's technologically possible or even an option for you access pattern, but for my more modern SATA disks I see about 1W (from the wall) per disk

Not sure if that's technologically possible or even an option for you access pattern, but for my more modern SATA disks I see about 1W (from the wall) per disk when spun down, lots of W for spin up and 5 to 8W spinning. I guess numbers for old disks are a bit different, but the principle should hold. Drawback is that you'll likely be doing staggered spin up, which can introduce some latency. But for non-interactive stuff like daily backups that's just irrelevant.