It's often said there's no such thing as a free lunch and that's doubly the case for free apps. A team from Purdue University found that nearly three quarters of the power used when you run an app like Angry Birds is actually used for adverts. It developed eprof, an app that investigates what processes are draining from your battery. Loading it onto the very old-school Android-powered myTouch 3G and Nexus One (not to mention a HTC TyTn II running Windows Mobile 6.5). Drilling down into those Angry Birds figures: the game itself only consumes 18 percent of the power, while advertising platform Flurry has 45 percent and GPS location tracking a further 15 percent. Project leader Abhinav Pathak lays the blame at the feet of poorly coded apps that need to be made significantly more efficient. He's now working with Microsoft to bring his software to Windows Phone and will present his findings at the EuroSys conference in Bern next month. If you don't think you'll be able to give up free apps, just remember to shut down GPS before you start smashing those pigs.