Purchased one of these on eBay as I am travelling a lot and wanted to see if this could be of any help. The thing that strikes me about this device is that it generates a power estimate purely from heart rate, without using FTP or any form of user-specific calibration in the algorithm. Other apps (HR2PV) use FTP, max- and rest-HR as input, and can understand how that is used to calibrate a HR-power curve. The Powercal apparently does it purely based on the shape of the HR trace. That is pretty remarkable, and makes you wonder about physiology. So muscle- and blood composition don’t seem to matter much, it is the heart muscle that predicts performance…?