Ive had some severe battery draining on a Glo HD. But that device went kaputt shortly thereafter, so it may have just been a hardware defect.
Ever since I've switched to my new H2O, battery endurance has been pretty good, and I've not noticed any unusual draining using the sleep cover with koreader.
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Anyway, I've made two minor interface adjustments that I find useful for the way I use the device. Posting it here just in case anyone else finds them useful. It's basically two additional icons in the reader menu. I just color-inverted the light and home button, cos I'm lazy. One allows for one-click light-toggling. I find that useful, since the H2O doesn't have a light button. And the other one is for shutdown. Which may be convenient for people experiencing battery drain in sleep mode.
The attached zip contains 3 files. The .png files go into koreader/resources/icons/, and readermenu.lua replaces that file in koreader/frontend/apps/reader/modules/. (Make a backup of the old version, so you can switch back if for whatever reason you don't like it.)
I just checked it out with the latest nightly build (20150808), and it still works. Cheers.
Edit:
See the updated version
here