I came across the Powercal Power Meter device when I was looking around for a power meter. It is a heartrate monitor strap that extrapolates power based on (I can only assume) change in heart rate using some form of algorithm. There is no direct power meter on a hub, ledal or crank - its only input is heart rate.

What a load of rubbish I thought.

Then I read the comparison below where the Powercal heartrate power monitor was compared with a real power meter.