Currently, the Auto backup setting is not available using a computer or mobile web browser. But, if you want to backup photos on your computer, then you can use 'Google Drive for your Mac/PC' by turning it ON. If you use 'Google Drive for your Mac/PC', then you should not use Google+ Auto Backup for desktop.
Google has two features to automatically backup photos i.e. 'Google Drive sync for your Mac or PC' and 'Auto Backup for Desktop'. If you use these two features together, then you might be taking up unnecessary space on your computer. To help prevent this, exclude the Google Photos folder from your syncing preferences in Google Drive for your Mac or PC:
Quick Tip to Backup your Digital Photos
If you have treasured photographs, or you take photos of everything you like, you won't want to lose them. So, what is the best way to backup photos on an ongoing basis? In the back of your mind, you know you need to backup all of your photos. But where do you start? Here's how to backup your digital life: PicBackMan is the easiest and simplest way to keep your videos safely backed up in one or more online accounts. Simply download PicBackMan (it's free!), register your account, connect to your online store and tell PicBackMan where your videos are - PicBackMan does the rest, automatically. It bulk uploads all videos and keeps looking for new ones and uploads those too. You don't have to ever touch it.