Probably a negligible amount. The accelerometer in HTC phones uses only 200μA when active, and only wakes itself when there is motion. I don't know exactly what

Probably a negligible amount. The accelerometer in HTC phones uses only 200μA when active, and only wakes itself when there is motion. I don't know exactly what part an iPhone uses, but it'll be similar. Compared to the CPU's power usage, which will definitely be running if the screen is on and the app is in use, that's nothing.