Metric guide
Power measures output, not the cost of producing it
The useful version
Use power to pace the work, then pair it with internal response to understand the athlete.
From average power to normalized power
Average power is literal and easy to compare. It can hide the cost of repeated surges, coasting and terrain.
Normalized power weights variable efforts to approximate their physiological demand. It is a model, so it should sit beside the actual power trace rather than replace it.
IF and TSS inherit your FTP
Intensity factor expresses normalized power relative to FTP. Training Stress Score combines that relative intensity with duration.
Both metrics change when FTP changes. If the threshold in your profile is stale, the precision of the resulting scores is misleading.
How to use the numbers
Use power for pacing, interval execution and comparing repeatable efforts. Use heart rate and perceived exertion to notice heat, fatigue and fueling effects.
Power zones are useful ranges, not walls. Day-to-day capacity can move even when the configured FTP does not.
Go one step further
Questions
What people usually ask
What is normalized power?
Normalized power is a weighted estimate of the physiological demand of a variable ride. It gives harder surges more influence than a simple average does.
What is intensity factor?
Intensity factor divides normalized power by FTP. It describes how hard the session was relative to the threshold value currently in your settings.
Do I need a power meter?
You need recorded power for cycling power analysis. Estimated running power is a different model and should not be compared directly with cycling watts.