The CycleOps Power PowerCal uses heart rate to calculate power and then sends both heart rate and power numbers wirelessly to any ANT+ device or ANT+ enabled head unit. Already have a Garmin but don't want to spend thousands of dollars on a power meter? The PowerCal is an effective solution for the budget-minded rider. It is also a great fill in for the PowerTap hub when you decide to use a non PowerTap wheel.
The PowerCal system is based on research done by CycleOps in conjunction with researchers at the University of Colorado Boulder. CycleOps and the University staff spent years reviewing the relationship between power output and heart rate. What they found is heart rate mirrored power in a very consistent and predictable pattern. They fed that data into the PowerCal system.
It works best for steady state efforts. If you are planning on doing intervals this would not be a good solution.