This has been covered extensively in the past.
Unless the Amazon engineers made a recent blunder - With WAN off, there should be little noticeable battery consumption difference between sleep and off. No additional power is consumed by the display system after a so-called "screensaver" is drawn.
At an abstract level - when WAN is off the on to off switch is simply a command to make the screen white and then take the system into a low power mode, instead of letting some time pass until the screensaver is drawn and the system automatically goes into the same low power state as "off".
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