That's what some first-of-its-kind research will reveal next week in a paper to be presented by a team of scientists from Purdue University and Microsoft at in Bern, Switzerland.

"It turns out the free apps aren't really free because they contain the hidden cost of reduced battery life," one of the Purdue researchers, Y. Charlie Hu, said in a .

Hu, colleague Abhinav Pathak, and Microsoft's Ming Zhang noted in their paper [] that free apps like Fchess and Angry Birds spend under 25 percent to 35 percent of their energy on game play, but more than 65 percent to 75 percent on user tracking, uploading user information, and downloading ads.

In their paper, the researchers describe a tool they've developed to precisely measure energy consumption within Android and Windows Phone mobile apps and show how that tool, called Eprof, can be used by developers to reduce the power consumption of their apps by as much as 65 percent.