You can set the power options so that it can shut down after a period of inactivity. Mine has 8 HDDs. According to my power meter plug, it uses 70W when all are spinning, 30W when the drives are spun down, and about 3W when "off" (configured to WOL). When I need to use the NAS for a while, I usually bring it up via an SSH app on my phone, which connects to my always-on Raspberry Pi that can send the magic packet. I use the wol package in Arch.