Tapes are offline and even require manual loading, so I think it's feasible to mitigate this by just powering down the backup system. At least that's what I do

Tapes are offline and even require manual loading, so I think it's feasible to mitigate this by just powering down the backup system. At least that's what I do (with my primary NAS). But yeah, disk idle usage should not be underestimated. Also, some nit-picking: Energy prices in Germany are currently MUCH higher than that. We moved and had to get a new contract. Close to 40c/kWh. This makes your point a bit stronger. //edit: Also2, when doing the math I realized I should first transcode suitable content to h265 (per TB saved the necessary power is cheaper than a new disk), and as a second step replace my four or five remaining 1 TB HDDs with a single bigger drive to reduce the idle power draw (the NAS is on a btrfs mixed-size RAID1).