A glimpse at the energy profiles of free mobile apps shows that their relationships to their energy source is quite complicated.

Abhinav Pathak, a computer scientist at Purdue University, Indiana, sought to answer the question, “Where is the energy spent inside my app?” In a study conducted in collaboration with his colleague Y. Charlie Hu and Ming Zhang of Microsoft Research, Pathak found out that as much as 75 percent of the energy used by certain free Android apps is spent as an aggregate result of serving ads and tracking user data. The study focused on apps running on Android, to be sure, but it’s safe to assume that the results also extend to the energy performance of the apps’ iOS counterparts.

Angry Birds consumes more energy in serving ads than in making the game itself work.

As it happens, only a small percentage of the apps’ total energy consumption is used in enabling them to do what they’re supposed to do, like providing high-quality journalism in the case of the free version of the New York Times app or hurling self-destructive birds towards the forts of their enemy pigs in the case of you-know-what. As pointed out by Touch Arcade and New Scientist, in a 3G-connected device running Angry Birds, only 20 percent of the allocated energy is used for the game’s core function while 45 percent is used in the process that ultimately leads to the delivery of location-aware ads. Pathak suggests that unless developers improve the third-party code that they use in programming their ad delivery systems, apps like the free version of Angry Birds will continue to be battery hogs.

While that free ad-supported app you enjoy using might not be deducting anything from your iTunes account balance, it might just be deducting excessively from your iDevice’s battery charge.