Performance & battery · Battery, power and Low Power Mode
Should I plug in my MacBook before a big move or a first backup?
Short answer: Yes, if you can. Big moves and first backups can run for hours and keep the Mac busy; on mains power they finish sooner without draining the battery. Interrupted backups resume where they left off.
It's a good idea. A large move or a first full backup can run for a long time — a first cloud backup can take hours — and it keeps the disk, processor and network busy while it does. On mains power it finishes sooner and doesn't eat into your battery.
It's safe either way: if your Mac goes to sleep or the battery runs out, a backup carries on from where it left off next time, and a move only removes the original from your Mac once the copy on the drive is complete.
See Moving folders and Backups.