How can I shutdown my PC when the system is idle?

Solution 1:

There are several options:

1. You could use a download manager application, they usually have an auto shutdown option. Search for one, where you can configure the idle time.
2. use a cron script: How can I shutdown the computer when a certain process ends?
3. configure your PC to go to sleep (hibernate) when there is no activity for a while
4. read the "related" box, right on your screen, and pick up one appropriate tip

Solution 2:

Edit: you can do this with https://launchpad.net/complexshutdown

If you're interested in how this works, look at How does ubuntu determine inactivity before suspending? for a good start. You can access the same counters that Ubuntu already uses before suspending, when suspend-on-idle is on.