iOS shows percentages related to battery usage in 2 different places. You can show (if enabled in settings) the percentage of battery power left in the upper right corner, next to the battery icon, something that has been in iOS since the early years.

Since iOS 8, you can also see which apps are causing the largest battery drain during the last 24 hours or 7 days.

I had never thought about it, but this is causing some confusion, as observed by Matt Henderson. In the images above, Tweetbot is not responsible for draining 21 percentage points of the 64 percentage points my battery is already. Instead, Tweetbot caused 21% of the total battery drainage over the last 24 hours. I suppose this number is directly calculated by dividing the battery drainage caused by Tweetbot in the last 24 hours (in some physical unit I don't know because I don't know or remember enough about electricity) by the total amount of battery drainage (in the same unit) in the last 24 hours. I suspect the same goes for the percentages in the "Last 7 Days" overview.

These two percentage numbers (21% for Tweetbot and 34% next to the battery indicator) are not directly related, but it's easy to see that one could think that. Can we come up with a better, less confusing way of showing the same information?

Tomas Verschoren suggests in a tweet to show the actual percentage points of drainage relative to the percentage number next to the battery indicator in the iOS status bar. Something like this:

While having the advantage of being visually much more clear, it has the disadvantage that it is related to the variable time passed since the last (full) charge. If your phone is (almost) fully charged, all these percentages will be tiny and won't give any useful information. Furthermore how do you calculate the 7 day overview in this way?

I believe that the percentages iOS is showing right now in the 24 hours and 7 days overviews are the right numbers to show, but maybe Apple could show them in a more intuitive / less confusing UI. Perhaps a good UI/UX expert has some interesting insights about this?