Let me help you with this.
Disabling the onboard graphics will not make a difference because the processor is still active. I mean to say that the onboard is inside the processor therefore the processor will always consume the same voltage even though the graphics controller is disabled.
The power consumption can increase or decrease depending on the devices connected in the NUC such as the SSD or memory but most of the times will be using the same voltage.
If any other user can provide feedback on this will be much appreciated!