To quote from ironman.com on the significance of average power and normalized power:
Let’s start with defining some basic terms. Average Power (AP) is simply that: the numerical average power output. In addition to looking at average power in a file, however, we also look at Normalized Power (NP). This takes into account the extra physiological cost of hard efforts—basically the times when you push hard to keep up with another athlete or up a hill. Essentially it is a measure of your perfect power score—the power you could have produced for the same cost to your body, if your power had been constant.
When you divide NP by AP you get a statistic called the Variability Index (VI), an important piece of data in our sport. This measures whether your power was consistent, or if you had spikes and lulls. For triathlon, the steadier the better, which means a perfect score would be 1…
What makes a “good” power file?
Here at TrainingPeaks, we’ve been able to gather power files from several top pros from a few different IRONMAN events. In each case, a strong file shows the following characteristics.
1. VI is 1.05 or lower. Pro athletes have a steady power output. A perfect case is 2012 IRONMAN USA champion Jordan Rapp. His power meter file showed a VI of 1.02 for the 2012 IRONMAN World Championship…, and a VI of 1.03 for the 2013 IRONMAN Asia-Pacific Championship in Melbourne…
What’s in it for me? You don’t need to be a pro to achieve a low VI. Since it measures how you put out the effort and now how much power is being generated, any athlete can have a low VI. Putting out a steady effort is much more efficient than spiking your effort, burns fewer calories, and means you can appropriately meter your effort. Even without a power meter, you can use heart rate or perceived exertion to limit surges and ride a steady pace to keep your VI low.