Originally Posted by
chaadster
They obviously cannot be completely unrelated, because you know, human physiology.
The real question is how much accuracy do you need?
PowerCal is a great example, I think. DCR claims it gets it in the right ballpark for average power over the course of a ride. But in terms of your power at any given minute, or even your maximum 20-minute average, it can be a few hundred percent off.