Idle - adjective
not working, active, or being used
not having any real purpose or value
not having much activity
idle power consumption is a measure of how much power is being used to run a device that has little to no activity.
To assume what the power consumption will be during a basic download, think about what it will actually be doing.
It will be communicating with the network interface, receiving packets and determining the info in them.
Then it translates that to bits of information, and sends it to the HDD to store the bits received in the packets.
This is a slow process in comparison to how fast a CPU can actually handle information, so I'd say it's safe to assume that the power consumption during this process will be close to idle, just slightly over.