Question about WD Elements Desktop

Hello everyone I have a Synology Nas at home as an external hard drive/server. Now I've been wanting a backup storage location for a long time and am thinking about buying the WD Elements Desktop with 18 TB. Now to my question: Does this external hard drive always have to be connected to the power supply or can it be disconnected from the mains after transferring the data and only be supplied with power again when the next backup storage is required? Does anyone know anything about this? Thanks for the answer.