W bal estimation method?

Curious and I couldn’t find this elsewhere on the site or forum after a quick search: How does the W’ estimate on the Power curve page work? Is it based on my previous/recent anaerobic efforts or “guessed” from my FTP based on some average relationship between FTP and W’?

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The default model is “Mortons 3P and FFT curves” which uses your best effort of at least 180s (configurable in /settings) to pick a predefined power curve model (CP, W’ and pMax). Those were created from a lot of empirical data from FastFitness.Tips.

You can click “Options” and change to “Mortons 3P” to estimate W’ etc. from your actual power curve data. This only works well if you have good inputs. You need 3 max efforts: 5 secs, 3-5 minutes and 12 mins or more (reference),

If you have good data then Mortons 3P gives good results otherwise its all over the place. Thats why the default is “Mortons 3P and FFT curves” which is much more robust. That is also what is used for the running eFTP estimation.


D’oh! I should have seen that. I forgot W’ was part of that setting. Thanks. As with everything here, that’s a cool feature. Thanks for all your work putting this whole tool together. I’m slow but I love data, so this really increases my enjoyment of cycling!


On the power curve I see W’ is calculated. On the individual workout page I see W’ is blank until I edit it? Have I missed a setting somewhere that will automatically populate the W’ field on the individual workout with calculated data. I also noticed that my W’ number is significantly different when I change to Mortons 3P, almost 20% higher. My coach says mine is high. Should I use Mortons? I probably have good inputs so based on your comment I assumed it would be more accurate to use Mortons and my actual power curve over FFT predefined curve?

You should definitely use Mortons 3P for W’ and not the predefined curve value especially if you have the correct inputs. You can enter your W’ in /settings (same line as FTP) and then it will apply to future rides.

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Intervals.icu now estimates W’ on an individual basis. You don’t need to switch to Mortons 3P. The estimation is a lot more robust with respect to the power inputs.


Just to clarify, I still need to manually update it in my settings, correct? Is there a way to have it automagically update, similarly to eFTP?

Yes. Also, it’s something you will need to pull from some other service, like TP or Xert…

You can get it from Intervals.icu now on the /power page. The W’ value displayed for a given curve is calculated from the data for that curve and not just based on a standard value for the eFTP. The circle on the power curve further to the left is the point used to calculate W’.


The problem I identify is that W’ is estimated from a model and sometimes the W’ value is not consistent with the data from a ride (Wbal can be push to negative values for some rides).

When my eFTP changes I first use the W’ value provided by the model as an initial value, but then I update my W’ value following an empirical approach. Each time a new ride push Wbal into negative values, I increase the W’ value to the minimal value such that Wbal remains positive.

With this approach I have a model that fits the efforts I can do.
With training my W’ increase until my eFTP increase which reboot W’ to lower value and restart the process.

I do it myself but it could be an automate process.


I missed that!

I do have automating that update on the todo list.


I have W’ set to 15.000 (default?) from the beginning on and never changed it.


I guess I should change that manually - right?

Yes. If you get W’bal to go negative on a ride and the effort wasn’t long enough to bump eFTP thats another hint.


Thx David, good one. Love intervals and the forum pretty much. Learning every day a little more :slight_smile:

It would be nice if when a new W’ is calculated, an achievement appears in the fitness chart, just like de eFTP

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