Originally Posted by RedHeadPeter
Well my understanding was that they use no power when not changing the display - has anyone ever tested this?
They do use power, even if minimal.
They have a RAM which must be kept "turned on", for example, otherwise at each page turn the device would have to
1) load the kernel
2) load the reading application
3) load the file
4) load the last read page
5) change page
6) turn off
Which is impractical.
The screen is, of course, the element of a PC or PC-like device that consumes the most (think of mobiles, netbooks and smartphones, the screen is the single element consuming the largest quantity of electricity) so the fact that the e-Ink screen consumes energy only when changing page improves significantly the duration of the battery.
I have made a couple of simple tests, and I noticed that if I turn off the device each time I stop reading, the battery of my device (a HanLin V3) lasts for about 10 days (reading 80 pages per day on average), while if I never turn it off it lasts only 6 days.
Of course, turning on and off your reader increases the energy consumption, so if you make a lot of breaks while you read you're better not turn it off every time you stop, or you end up sucking even more energy from your battery instead of saving it. Turning it off helps if you make long reading sessions with no breaks.