We tested the peak power consumption of a NAS at the wall during a run of CrystalDiskMark 5.0.2 as this version of the benchmark runs the read and write benchmark suites separately so it is easier to monitor what power the device is using during each function.
The official power consumption figures for the DS120j are 4.68W in HDD hibernation and 9.81W when being accessed. These figures are with a 1TB WD Red (WD10EFRX) installed in the NAS. The 6TB WD Red we used for testing the DS120J has slightly higher power ratings (Read/Write 5.3W, Idle 3.4W) than the smaller drive, as you can see when looking at our power consumption chart.