As iCloud Photo Library is now in full operation, I wondered if it was possible to somehow back up my whole iCloud Photo Library. I had a look at the settings of the Photos app on OS X and found the that there’s an option to Download Originals on this Mac;
Store original photos and videos on this Mac. Choose this option if you want to access full-resolution versions of your entire library, even when offline.
In order to find this option, go to Photos, Preferences, iCloud:
That seems to do the trick. Looking at the ~/Pictures/Photos Library.photoslibrary file, mine is about 90GB, which is roughly what my iPhoto Library was before I uploaded it to iCloud. Scrolling through Photos seems to display all my photos too, so the transition seemed to work just fine, but having an offline backup may come in useful one day. Once the Photos Library if on your local disk, it’ll back up along with all your other files with Time Machine.