Light off and sleep are not the same, and an idle kindle with sleep disabled will deplete the battery significantly sooner than if sleep is enabled.
https://www.mobileread.com/forums/sh...d.php?t=138778
Battery usage by the light varies from a small fraction of idle power to more than everything else combined, depending on brightness setting.
https://www.mobileread.com/forums/sh...d.php?t=293683