You are correct that the screen curtain in JAWS will not save significant battery on a laptop since it only turns off the actual display, but does nothing with the backlight (which is actually a edge-spill light, but that doesn't really matter). You have a lit black screen.
Just activated it on/off with INSERT+SPACE,F11 and it's clear that the backlight remains on. Whether it remains on for hours I cannot say, and I cannot use my computer without looking at the screen.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
The purpose of education is not to validate ignorance but to overcome it.