My NAS/home server takes about 180 kWH/year, or 20W on average (costs me about 5 Euro/month). This feels like a lot to me and makes up a good chunk of my total energy usage.
What I did to optimize it is mostly select a “low-power” x86 platform, and limit the number of disks. I also decided not to use TrueNAS because it couldn’t let the disks from going to sleep.
The machine hosts 10 TB of usable storage (+RAID 1), syncthing, immich, file server, Influx DB, Grafana, and a few other services.
I think for me spin-down was working initially. But then I added some Docker containers and those could only be run on the HDDs, blocking spin down. With Debian I just put the containers on the smaller SSD, allowing the HDDs to spin down.