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.
I use a KillaWatt device. It is a simple device you plug into the outlet and then plug the device you want to measure into it. I’ve had mine for a long time so I have no idea what a new one would cost, but I’m guessing sub $30.