There's also the fact that hard drive capacities keep increasing and increasing significantly faster that the power required, and sooner or later for very long term storage it'd become cheaper to migrate all your data from those 5 year old 4TB drives to more modern 16TB ones. That's assuming you want hot access to the data and don't plan on spinning them down as soon as you've written to them, like you'd do for a cold backup of the whole IA.

I remember for a long time (I'm talking 20-ish years back here), every hard drive I bought had double or more the capacity of every drive I'd ever bought previously combined. My first ever 40MB (yes, megabyte) drive got upgraded to an 80MB one, that got updated to a 250MB one, then a 750MB, and then a whopping 2GB drive (how would I _ever_ fill that up???) - and so on. That's slowed down some, but I'm currently starting to think about upgrading my 8TB drives (Raid1 pair) with 20TB drives when the prices start to drop a bit more.