If you’re trying to save power as a cost savings measure, then you should also consider that wearing out a disk prematurely means paying more money for disks because you have to replace sooner. If you’re doing it out of an environmental concern, you’re still better off trying to prolong the lives of the disks as the carbon emissions from their manufacture while outweigh whatever emissions you save from spinning down your disks.
That said, this is entirely dependent on the question “does spinning disks up and down shorten their lifespan?” The answer is generally yes, but there are certain rare scenarios where that might not be true.
If this data is only accessed once in a blue moon or so, then it might make sense to spin down the disks in that time. But this is probably not your use case.
And regardless, any power savings you would get would be minimal. You’re better off trying to figure out a way to better cool the system if you want to save on power.