Mac storage basics · Time Machine basics
Should I encrypt my Time Machine backups?
Short answer: Yes, if the drive could be lost or stolen. Tick Encrypt Backups when adding the disk, and store the password safely — you can't restore without it.
Yes, if the backup drive could be lost or stolen — it holds a full copy of your Mac. Tick Encrypt Backups when you add the disk in Time Machine settings (or encrypt the drive in Finder).
Keep the password somewhere safe: without it you can't restore. Encryption doesn't meaningfully change how much space backups use.