I have been using an old PC as NAS and running docker containers (Immich, Nextcloud, paperless-ngx etc.). I got 5 3.5" HDD disks and a Nvme. Even on idle, I am consuming 50-60w. A friend of mine is selling a Qnap NAS which is a dedicated machine and probably consumes less power, although I don’t know if it’s worth it.
Some local libraries (e.g. in Heidelberg) or ecological initiatives lend devices to measure electricity consumption at the power plug. In particular, this is useful to measure other appliances as well.
Specifically for computers, they probably have some means that tell you their own consumption, but they may not be accurate or complete and will most certainly omit any peripherals, e.g. external hard drives.
Thank you! I will see if I can get my hands on one of those.