Q: Re-install El Capitan without Re-download

Hi, take a look at this article How to reinstall OS X on your Mac - Apple Support

Factory settings return your computer to the state it was in when it was purchased.  You can't return your computer to factory settings AND keep El Capitan unless your computer came with El Capitan when you bought it.

Format a spare USB thumb drive, making sure to name it "Untitled".

Then open a Terminal windows and paste the following command:

​ 

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction

After this, you can boot with this drive by pressing the ALT key at startup, erase your entire hard drive and re-install El Capitain without re-downloading.

Hope this helps!

that only works if you have the installer in it's default location and if you already installed it then it deleted itself and the process will fail

see the complete details here

Create a bootable installer for OS X - Apple Support

You can make a bootable USB stick to install using this free program which will do all the work for you.

Bootable USB Flash Drive – Diskmaker X

DO NOT USE Diskmaker X - unless you don't want AirDrop and continiuity to properly

DiskMaker X - (Airdrop doesn't work properly?)