Well, my slim CanoScan 1220 (1240? I forget) has no separate power cord, just a USB cable. Plug it in, and it waits in a low-power "sleep" mode. Start a scan, and it wakes up.
Want to turn it totally off? Just unplug the USB cable.
It's a nice scanner. Not the fastest (a limitation of USB, I suspect), but plenty fast for my uses, and a great slim profile and easy use.