You're considering buying a power meter and all you want to do with it is look at some number in Watts with whatever random app you can download for free? I suggest putting a bit more thought into what you actually want to do with those numbers, like recording then for training and/or testing (fitness, aero) and/or monitoring power over the course of a ride for pacing. Then you can come back and ask what free (or even paid, possibly better) apps are out there that can show and record the sensor data, load interval workouts, analyze the power distribution of your rides, or estimate your drag coefficient.