Filed under "you get what you pay for": Researchers at Microsoft tested the energy consumption of various apps on Android and Windows Mobile (iOS restrictions kept them from testing the iPhone) and found that up to 75% of an app's energy use goes to power the advertisements in free, ad-supported apps.
The numbers may not have much of an impact without tying them to an app you actually use, so what about an extremely popular game like Angry Birds:
In the case of Angry Birds, research suggested that only 20% of the total energy consumption was used to actually play the game itself.
Of the rest, 45% is used finding out your location with which it can serve targeted advertising.
The lesson: Next time you're choosing the ad-supported version of an app over, say, its $1 counterpart, you may want to consider whether you're willing to pay extra just to keep your battery going longer. Yet another way free stuff isn't always worth it.