… I recommend creating your own bootable El Capitan (OS X 10.11) installer drive on an external hard drive or USB thumb drive. If you need to install El Capitan on multiple Macs, using a bootable installer drive is faster and more convenient than downloading or copying the entire installer to each computer. If you want to erase the drive on a Mac before installing El Capitan, or start over at any time, you can use a dedicated installer drive to boot that Mac, erase its drive, and then install the OS (and subsequently restore whatever data you need from your backups). And if your Mac is experiencing problems, a bootable installer drive makes a handy emergency disk.
What Dan said. I’ve been following his advice for years, as he’s updated this article many times. Make one as soon as El Capitan comes out, and it might save you a lot of time and trouble.