Carat is an interesting idea that has been tried before and met with minimal success. That is because, despite users running the same apps across the same platforms, each device responds to these apps differently. A bug on one Android phone may not affect another, and will interact with an errant app in a completely inconsistent way, activating GPS or WiFi when not necessary, for example.
Carat uses crowd-sourced data from thousands of users to determine which apps have the tendency to use more battery in the long run, either when in the fore (such as ad-supported apps) or the background (such as those that continually use CPU resources or activate the GPS).
It’s not a fool-proof system — the app will make a report on your smartphone to determine which apps it thinks best to close — but it could become an essential part of a users’ repertoire when trying to eek out every last drop of power from his or her battery.