Can you turn the computer on and off with cron jobs? I don't think it's enough just to stop mining, even idle cards can pull something like 30W.
you can turn it off with cron. back on could be done simply with a setting in the bios to power on @ certain time. that would at least avoid the inevitable filesystem corruption and other issues that stem from simply cutting power to a running linux box over and over.
however, I strongly recommend you *do not* implement a scheme of powering your mining rigs on and off every day.
a vast majority of the USB key wear occurs during boot up. you will accelerate the death of your usb keys tremendously. if you're so worried about power that you can't run a box at idle, you're in the wrong hobby (or the wrong location, at very least).