I check and log my smartphone battery reading at 10:00 each night and calculate % of battery consumed per hour.
Here's a chart of my findings on my iPhone 6.
I don't claim this to be scientific, just interesting.
iOS 11 introduced a significant increase in battery consumption then something was changed in iOS 11.2.1 that noticeably reduced battery consumption.
As the phone ages, the battery capacity reduces thus increasing the % per hour due to a smaller base. I didn't include this in my logging as until iOS 11.3 Apple didn't expose the battery capacity.
Incidentally, my battery capacity was 87% on the first of February and 86% on the first of April.