Variation in battery is definitely normal. Apps can, at some point in time, request more data/resources throughout the day, different than other points in the day. I used to be super worried about battery longevity because I was a super light iPhone user. I could get 5 days out of my iPhone 4s and 5 if I wanted to. Helped that I worked next to a cellphone tower.
But -Gonzo- is right. A cellphone is meant to be used. I got a few Anker batteries, put a charger in every room, a charger at work, now I can use my phone without worrying about whether or not my phone will last all day. I charge it all the time and it spends most of its life 90%-100%.
That said, if you go to the Settings --> Battery, it should tell you what is taking up your battery. Also, use the process of elimination. No one here can tell you about YOUR phone in front of you. Only you know what you did to your phone, what apps were installed or not, if your phone spent a few days in the hot sun in a car or not, or your reception is now 1-2 bars whereas it used to be 5 bars. There are a lot of factors in play that can affect battery life. You must find it out.
I find using coconutBattery (if you have a Mac) very useful in gauging my battery health and life.
https://www.coconut-flavour.com/coconutbattery/
I find hard booting my phone (on the 8+, do volume up, volume down, then hold power button for 4+ seconds) once a week very helpful. On reboot I usually close all apps by doing the home button double tap, then swipe up to close app. I don't do this as often as I used to (now, once a month or less).
I uninstall apps that I don't use more than once a month and don't need. I don't do Facebook or Instagram or a lot of talking on my phone and this seems to have the largest effect on my battery life vs my wife's iPhone 8+.
