An out-of-date FTP quietly ruins a training block. Set it too low and every workout is easier than intended, while setting it too high has you failing intervals for weeks and concluding you are going backwards.
WattPlan watches for both your threshold and your anaerobic capacity moving, and the rules behind that explain why it sometimes says nothing after a ride that felt enormous.
The twenty-minute route to FTP
The classic estimate takes your best twenty-minute power and multiplies it by 0.95, and WattPlan applies the same arithmetic to the best twenty minutes in a ride.
It prompts when the resulting figure is at least three per cent above your current FTP. Below that, the difference sits within the noise of a good day, a cool room or a fresh set of legs, and changing your zones for it would be churn rather than progress.
It will not change your FTP on its own, only show you what it found and ask. It also will not lower your FTP because of one disappointing ride, because a bad day is not a fitness loss.
The curve route to critical power and W′
Separately, WattPlan estimates your critical power and W′ from the shape of your power curve, using the Monod two-parameter model fitted by linear regression across durations from one minute to twenty. Critical power is roughly the effort you could hold for a long time, and W′ is the finite amount of work available above it. There is more in W′ and W′bal explained.
For this it merges the current ride with up to ninety days of history, taking your best at each duration, so a single strong ride does not have to carry the whole estimate.
It prompts when critical power has moved by five per cent or W′ by ten per cent, and only when the fit is good enough. The estimate carries a confidence rating derived from how well the regression fits and how many durations contributed, and a low-confidence estimate is discarded rather than shown.
Why it stays quiet after a sweet spot session
A critical power estimate needs efforts at genuinely different intensities. A steady session gives you one intensity repeated, and fitting a curve through that produces a confident-looking number that is essentially invented.
So before estimating, WattPlan checks the ride has enough variation to be worth fitting. Your one-minute power has to be at least eight per cent higher than your ten or twenty-minute power, and your best short effort has to exceed 102 per cent of your current FTP, and a ride that fails either test is skipped.
In practice it looks at VO2 max sessions, threshold work, tests and races with attacks in them, while ignoring sweet spot, tempo and endurance rides, which make excellent training and terrible measurement.
Why it asks rather than acts
Changing your FTP changes every workout in your plan, which is a large change to make on the evidence of a single ride.
Decline the prompt when the ride was unusually short and sharp and the estimate has extrapolated from it, or when your power meter has just been recalibrated or replaced, or when you rode a new trainer for the first time. Decline it too when the number is flattering but you know you could not hold it for an hour, because you will spend the next month failing intervals.
Accept it when the number matches how the last few weeks have felt, when it comes from a real test, or when you have been finishing sessions with something left.
One limitation worth knowing
FTP detection from a ride needs the full second-by-second power data available in the browser. Rides that arrive from a connected service as a summary may not carry that detail, so the prompt is more likely after a ride recorded in the app than after one imported in the background.
If you want a definitive answer rather than a detected one, ride a test. The library has them, they are built so you can exceed the target rather than being held to it, and twenty minutes of honest discomfort resolves the question for the next couple of months.