'Typically, a modern phone battery's (lithium-ion) lifespan is 2 – 3 years, which is about 300 – 500 charge cycles as rated by manufacturers. After that, the battery capacity will drop by roughly 20%.'
I get the same problem and think Johnny is on the button. I live in a poor signal area and my phone runs flat with little use. I know it's not the battery because I have taken the phone elsewhere, even abroad and it lasts for 24 hours or more.
I would also check you don't have apps running in the background, turn off bluetooth and mobile data when at home and yes check your battery usage in settings.