[robko]

Just to clear up some potential confusion here: with the older Kobo Wifi (as opposed to the Kobo Touch) there was the option to have the Kobo wake up and automatically download your newspaper in the morning. This could eat a lot more battery for people who didn't have any newspaper subscriptions anyway, so it was recommended to turn this off. The Touch does not have this option at all. Powering the Kobo off would prevent this from happening (you had to leave it in sleep mode). Putting the Wifi in airplane mode (as mentioned previously on here) is the equivalent of turning it off. Yes there must be a tiny amount of battery being used to keep the time, but I can say from experience that my Kobo Wifi in powered off mode will last for several months or more with no appreciable change in power level on the battery (I use my Touch these days).
With the Touch if you don't have the wifi in the airplane mode, you'll notice it automatically connects and updates your bookmark positions when you open/close a book (at least I assume that's what it's doing) so it will use more battery.
And power off puts the whole unit into power down mode. The screen is pretty much in power down any time it's not changing something on it. When the unit is not powered down it is using energy to maintain the data in the volatile memory (the book you have currently loaded to read as opposed to the books stored in the non-volatile long term memory) and polling the touchscreen waiting for touches.