Powergremlin provides iOS6 device power usage measurements.
It only runs on iOS6+. You'll need XCode4.2+ and a valid iOS developer account to run the code on your phone. The code itself has no external dependencies. To get started just do something like this:
git clone https://github.com/palominolabs/powergremlin.git powergremlin cd powergremlin open PowerGremlin.xcodeproj
Then connect an iOS device and run! If you connect the phone to iTunes you can copy logs to your computer so that you can see the usage over time.
For more info check out the Palomino Labs Blog