My understanding of it (and anyone who knows better feel free to correct/ ridicule me) is that a constant load is put on the power source to power the displays and processors and the amount of ambient light has little to nothing to do with the amount of power consumed.
NVGs don't make light, they simply amplify already existing light and the "brightness" of the image seen by the human remains the same. The NVGs simply adjust the electronic aperture to allow an appropriate amount of light into the gogs much like a human pupil.
Hope that helps!
