Originally Posted by
Iride01
But what is the kiloJoules representing that was put out by the power meter? Is it simply a straight conversion of total watts to kiloJoules or kilocalories? Or does it represent the amount of Calories that the body burned to put out that amount of work?
...
The reason a power meter is useful for training is it reports what was delivered to the bicycle. Not what your body produced to get it there. A dietary Calorie total will vary depending on all sorts of factors.
1 watt = 1 joule per second.
The total kJ or mJ reported from a direct force power meter is extremely accurate. There isn't a better source.
Converting joules to dietary calories (which are exactly the same as exercise usage calories) has a maximum error of 5% and the way it's most commonly done puts you in the middle of that range. Yielding a maximum error of +/- 2.5%. Over the course of a 2,000 kJ ride (mountainous half century in my case) that's one Oreo of uncertainty.
The only way to get better accuracy of dietary calories than a power meter is to use a metabolic ward.
Good heart rate monitors are very accurate at counting heart beats. But how many heart beats are there in a calorie?