Battery usage is best thought of as a sanity check. It lists each app (or service) you’ve used in the last 24 hours or 7 days, not including apps used while charging, along with the percentage of power drain they’ve been responsible for.
Since about a week ago —before I upgraded to iOS 8—, I’ve noticed a sharp reduction in battery life on my iPhone 5S. It doesn’t last into the evening, while previously it lasted a whole day with power to spare. It’s weird, particularly because I’m not using it significantly more than I always have, and I’m yet to find a concrete cause for this. Maybe my iPhone’s battery is damaged, or maybe there’s an app that’s misbehaving and draining it a lot quicker than it should. I don’t know.
The battery usage monitor, which is a new iOS 8 feature, is helping me find the culprit. For example, I thought Overcast would probably be using a lot of power, but it turns out it isn’t. That’s a relief, because I really enjoy using it. Similarly, Maps and the Phone app are the two most power-hungry apps in my phone, both due to “Low Signal”. Now we’re getting somewhere.
I think it’s worth it to spend a while getting used to this feature. There are some important things to learn about how we use our phones, and what we can do to make them last longer.