Why does MacBook Pro battery discharge when not in use?

I generally keep my 2018 MacBook Pro plugged in and I use it casually when not plugged in throughout the day.

With light email checking in Safari throughout the day, I have gone from 35% to 13% today. This includes large jumps that decrease while the computer is closed.

Because I am aware of this problem, I tried closing the lid when not in use, also Power Nap While Using Battery is disabled.

My previous computer was a 2015 MacBook Air. I used that in a similar way and was able to use it entirely untethered from power for the entire week.

How can I diagnose battery usage for the times that the computer is closed?