Q: I heard from others that battery life is lesser than it was when they were on Yosemite... Can anyone tell me if its actually true? And is there a possibility to downgrade to Yosemite once i upgrade to EL Capitan? This is my first time with a macbook

Bhaskar Datta wrote:

 

Want to know about battery life and downgrading to Yosemite.

You can only downgrade your operating system if you a) have a Time Machine backup from when you last ran that OS or b) previously downloaded that OS and can access the installer through the Mac App Store and your Purchases tab.  Otherwise, you cannot.  As for battery life, my computer is 5 years old, so my battery is 5 years old, therefore it does not perform like a brand new battery, but since upgrading to El Capitan last year, I have not noticed a marked or obvious decrease in battery performance.

I heard from others that battery life is lesser than it was when they were on Yosemite... Can anyone tell me if its actually true?

It is usually just the opposite case.

you will need to currently own Yosemite, either purchases with your iCloud account and available on the app store or having it on some install form like a flash drive or a disk image. if you do not have it already it may no longer be legally purchased from Apple

You will need to install the Yosemite image to another device as running it from the same drive currenty running a newer OS will prevent you from downgrading.

Create a bootable installer for OS X - Apple Support

to downgrade to the same drive as your current OS you should backup that drive with time machine, the format the drive, then downgrade.

Alternately if you partition the drive or use another drive you can install without formatting, but a backup is still advised.

as for battery life it is dependent on so many factors that singularly changing an OS may have more to do with the state of the battery, the power consumption of the laptop, HDD vs SSD, services running, applications running, wi-fi vs. wired, connected devices, etc.