Research by Abhinav Pathak on free Android apps shows that up to 75% of the energy that they consume is purely used to show ads. As an example, Only 20% of the battery drain from playing Angry Birds actually comes from playing the game and running the screen. The other 80% is split between finding the user’s location and another 28% is used up uploading the data to the internet. The remainder is mostly miscellaneous things that vary from moment to moment.
After studying the apps, Pathak concluded that only 10%-30% of the power consumed by free apps actually goes towards running the game, and that these results apply to most ad-supported games.
So if you are struggling to get through a day on a single charge of your battery, you might want to look at your apps. If you tend to use any ad-supported ones, well, you could be doing in the battery performance of the device yourself.