In addition to power usage, I measured how much the NAS heats up my server closet compared to the closet without the NAS. It’s not exactly scientific, but in my 3’ x 5’ x 8’ closet, the NAS increases the temperature 2˚F from 73˚F to 75˚F. The fan spins at its slow speed all the time and is supposed to speed up if the unit gets hot. I’ve never had my NAS get hot enough to require the fan speeding up.
This NAS is not exactly silent, and since you may be using this unit as a media server, you might be interested in its sound level. At the distance of about a foot, the unit produces about 40dB while the drives are spun down. With the drives awake it registers 44dB. While the NAS is performing a synchronized backup to an external drive, you can hear the drives accessing and the sound meter reads 46dB. By my more subjective ear, it makes about as much noise as a mini tower computer depending on the amount of drive access.