If you’d like to save some space (and a bit of time, especially on your initial backup) with your Time Machine backups:
- Open a Finder window and go to your Mac’s hard drive
- Open System Preferences > Time Machine, then click Options
- Drag the System folder from your Mac’s hard drive into the “Exclude these items from backup” section
OS X will prompt you to exclude other related items, including system applications. You probably don’t need those core items since they come preinstalled in OS X anyway, and your initial and potentially future backups won’t take as much time.
You’ll save space on your Time Machine backup drive, too. Once I enabled this option, the total excluded file size was 26.82GB.
Good point from Kevin Miller in the comments:
A very important caveat: once this is done, you can no longer restore a bootable system from scratch using Time Machine. You can only restore individual folders.