I saw this story on Velonews today: Upcoming from CycleOps: Heart-rate-based power meters and superlight carbon wheelset I am going to say this as clearly and simply as I possibly can. There is no way to measure power using heart rate. It just isn't possible. Heart rate is to dynamic. It changes due to factors such as fed state, level of fatigue, how long you have been exercising, hyrdation level and more. Power measured via strain gauges is instantaneous, and any effort put into the pedals can be measured right then and there, but but heart rate may take several minutes to catch up. So heart rate is a reasonable proxy for effort level on long efforts, but is practically useless for long efforts.
Like I said, I love my powertap, I have one on my bike, and I have a studio full of powertap stationary trainers. But don't tell us you are measuring power when you aren't. What PT is trying to do is measure training stress using heart rate, and make it applicable across different types of exercise. This is nothing new and it can be useful. You can read a good article on TRIMP HERE. But there is absolutely no no way that TRIMP equals power measurement.