My (analytical) experience with indoor vs outdoor watts

Hey all - I thought I’d share my experience with indoor vs outdoor watts. I’ve done a lot of the reading on the topic, so I don’t necessarily have a question but I found this interesting. (Edit: it turns out I do have a question at the very end.)

I started the 18-week sweet spot plan in early January, so I’m a few months into it, and I’ve got a good feel for what I can and can’t do. I’ve also been doing this a few years now, so I have an intuitive feel for how hard sweet spot should feel. Anyway, I started out the training plan using my ‘outdoor’ FTP on Zwift, and it’s HARD! My sweet spot intervals call for me holding ~275W, but it’s usually a struggle to finish them all at 250. Even at those lower wattages, my heart rate seems high for my sweet spot efforts, coming in around 170 or so.

Outdoors, however, it’s the opposite. Granted, I was feeling good yesterday, but I crushed some longer sweet spot intervals, easily holding 299 and 300W on all four 12-minute intervals. Heart rate was pegged at 163-164 for all four as well. So being a nerd, I decided to dig into the data:

Yesterday and one other recent outdoor ride seemed consistently show ~300W and 163bpm for my sweet spot, without overdoing it - it felt like a solid sweet spot effort. I’ll use those values for my actual FTP (currently around 325W).

To compare, I plotted my indoor interval efforts - these are all average power and average heartrate for a bunch of sweet spot and tempo intervals:

indoor power

If I plug in a heartrate of 163 into that equation, I get a power output of 243W, which honestly feels pretty equivalent to those 300W outdoor efforts! Now, I know that my on-bike power meter is about 3% higher than my Kickr, so let’s say the actual indoor power is 250W. That’s still almost a 20% difference for the same perceived exertion. I do have pretty decent fans and a window nearby to let in wintery air, so that’s definitely not the whole reason. I built up a rocker plate which maybe is robbing me of a little power (but so worth it for the comfort). But otherwise I guess it comes down to real road feel?

Oh, I guess I do have one question for the team: even if an indoor workout at 243W is more or less equivalent to a 300W outdoor workout on the cardio level, I’m assuming it still more beneficial for my legs to workout at the higher wattage outdoors. Is that a good assumption?


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I have a kickr core, and frequently had trouble with it being 20% or more off from my stages PM, so I eventually threw in the towel on the kickr power, and always use the stages PM. Prior to this solution, I was using two different FTPs one for indoor and one for outdoor. But now that I use the same PM indoors and outdoors, I don’t see the need to have separate FTPs, even though outdoors feels a bit easier than the trainer.

That’s my 2 cents on it.


Yeah, that’s what I was thinking too, so I rode with my power-metered bike on the Kickr and determined that my Stages is reporting only about 3% higher than the Kickr, which is basically a wash. Anyway, that’s why I was so surprised to find that the rest of the difference is apparently the result of other factors!

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