The nominal power consumption of a Yubikey 2 is about 10mA at 5V, so this should be about 0.05W. If the power consumed in the Nano would in fact have been 8W, it would have been on fire for sure.
If the readings are correct, I guess this rather has to do with the power management in the computer. With a USB device attached, it may not be as aggressive in shutting down and larger parts of the USB subsystem may be enabled. Just a speculation.
However - 8W for the device itself is completely out of question, unless you see a plume of smoke coming out there.
Best regards,
JakobE
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