You may have noticed that in some intervals your average power is lower than your normalized power (NP). How can this be?
Normalized power is generally considered invalid for durations less than 20 minutes. It used to be hidden in both WKO and TrainingPeaks, but we had a great deal of people asking to see it despite the invalidity of the data.
Let's say you did 1 minute very hard and 1 minute off, repeating for 10 minutes. The normalized power would weight the "on" intervals so heavily that it would be an unrealistic report of what you could do for 10 minutes. The measurement doesn't become an accurate representation of the physiological toll until at least 20 minutes.
For very short intervals, normalized power can be lower than average power. This is because that when calculating normalized power, each data point in the selected range is calculated using the entirety of the 30 seconds prior to itself, which is the basis of NP. The accumulation of stress in your muscle at any point in time is affected by what has happened in the prior 30 seconds of any data point in question.
For example, if you were soft pedaling before a 20-second sprint, your normalized power would be lower than average for your 5-second power. This is because the first second of your sprint incorporates 29 seconds of soft pedaling and 1 second of sprinting; the second second of your sprint incorporates 28 seconds of soft pedaling and 2 seconds of sprinting; and so on. This will of course result in a lower average than the 5-second average. The same is true for a 30-second peak or a minute.
The reason we use a rolling 30-second average is that it takes 30 seconds of an effort before our heart rate catches up. The idea is to mock heart rate as Normalized Power as an estimation of the physiological toll of the work you have done in watts.
If you use a Garmin Edge device, you can have normalized power displayed on the screen, but it will not display anything until after 30 seconds have passed, because NP has to have that 30-second range prior to the duration in question.