1 HDD consumes around 5-7W idle, so with 8 HDDs you get to 40-60W on HDDs alone (all idle); adding 6W with N100 seems like insignificant fraction. The moment you actually use any HDD the consumption per HDD shots up to 8-10W whereas N100 shots up to 14W so you end up with 64-80W from HDDs and 14W from N100. Why would you like to squeeze component that is the least important (CPU) while retaining lots of SATA HDDs as that's your priority? Optimizing the wrong thing? If you wanted to lower power, the easiest way is to replace HDDs with 16TB SATA SSDs, each consuming 0.08-2W. Then CPU might be a bottleneck.
For my typical usage, the hard drives are probably more than 80% in sleep mode. If I had more SATA ports, I could probably add a frequent-access cache on a SSD and then they would be 99% sleeping.
The drives I have, ST2000DL003, consume 0.5W in sleep, according to the spec sheet. So all 8 of them would consume ~4W.
In a RAID6 with 8 drives, you can allow one disk to go offline and remain able to write, or two disks offline and remain able to still read 100% of data. You lose 2 of 8 = 25% capacity on redundancy.
With a mirror of 2 disks, if one disk dies, you can still read; if two disks die, you're toast. And you lose 1 of 2 = 50% of capacity on redundancy.
Because TCO is less with these cheap old drives and I can have RAID5 instead of just RAID1 or no RAID at all.
But you're right. In a few years it will become advantageous to switch to a couple of larger HDDs. I could probably do it right now, but I don't yet trust these new drives as much as I trust the old ones, especially since the refurbished scandal.
1 HDD consumes around 5-7W idle, so with 8 HDDs you get to 40-60W on HDDs alone (all idle); adding 6W with N100 seems like insignificant fraction. The moment you actually use any HDD the consumption per HDD shots up to 8-10W whereas N100 shots up to 14W so you end up with 64-80W from HDDs and 14W from N100. Why would you like to squeeze component that is the least important (CPU) while retaining lots of SATA HDDs as that's your priority? Optimizing the wrong thing? If you wanted to lower power, the easiest way is to replace HDDs with 16TB SATA SSDs, each consuming 0.08-2W. Then CPU might be a bottleneck.