kindle displays don't use power like laptop screens. It uses very little power to sustain an image, and only uses more power to change the pixels.

You can keep the display of the kindle on for hours at a time with minimal drain

One of the key features of e-ink is that it actually requires zero energy to sustain an image. It only consumes energy for state changes between images.

Of course, modern e-readers have a backlight. Though you can never fully turn it off, at least on my first-gen Paperwhite, which is unfortunate.

Definitely going to be ereader-specific, my Kobo has a physical on/off button for its light.

Yes, that's why a wifi card being comparable to a laptop screen demonstrates how power hungry they can be.