Short answer is likely higher, but it’s hard to know how much. Every person and situation is different and the only way to really know is to test in both locations.
But why? Why not get a good fan, help your body with the key evaporative cooling, and then even more of your energy can go into performance rather than trying to keep your body’s systems from overheating? It’s pretty well established.
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Another possible factor causing indoor differences for some of us (depending on equipment) may be trainer inertia. While all the talk is about erg trainers and the assumption that one can adjust inertia with gear changes, many people are on dumb wheel-on trainers and the inertia is more fixed based on flywheel (and likely lower).
So with higher inertia outdoors you can do more power at the same RPE vs. lower inertia indoors.
Feel like the TR gang whenever they discuss this topic (many times) talk only about cooling and indoor practice (both key too of course) because their frame of mind is smart trainers, so they don’t have to account for lower inertia.
To wit, I’m pretty sure I remember Coach @chad once upon a time in the podcast specifically advising against doing an outdoor FTP test on a sustained climb because it’d be lower inertia.
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…Or it could be lower outside…
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Sure, totally possible. But based on the conditions listed and the fact that most people perform better outside leads me to expect some increase in this case.
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Dumb trainer here…so that can be the thing…
That is a very good point. Do you adjust the Outdoor TSS by the factor Indoor TSS/Outdoor TSS - assuming you work with your indoor FTP normally?
I wish I was one of those with a higher outdoor ftp, but I’m fairly sure I don’t, although that whole cognitive load thing might apply to me outdoors, as there’s only one spot in my area (actually down near my parents’ house an hour away) that I can do a concentrated power effort for 20+ mins. I would be thrilled if I could be 20-30w higher outdoors lol
The closest I’ve really come to testing myself outdoors was this past summer when I was in Portugal and had a mountain near my parents’ home there. I did a 20min NP of 285 (my FTP then was set at 275) and I probably could have gone harder and for a longer time, but the temps were in the 50s and rainy (the descent was chilly!). I did a couple of other efforts up the mountain while I was there but they were more reasonably paced at like 265w NP, looking back I wish I had really done a full gas effort (either 20mins or the full climb)
Good point. But also consider some people are better at high interia pedaling (TT), while others are better at low interia pedaling (climbers).
Here’s a very interesting article on this very topic:
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Interestingly my Torque Effectiveness is outdoors between 90 and 95% while indoors between 75 and 82% (for power ranges around threshold and measured by Favero Assioma for both instances)…
Did anyone come to a conclusion on this? A ramp test on Sunday had me at 260W (4iii left side) yet an 1.15hr (18mile) commuter ride this morning had an NP of 280w (IF 1.09). In terms of TSS it didn’t feel like 124. Having 4 250 TSS commute days added to my calendar is really messing up CTL charts vs a winter on the trainer where 500 TSS was about as much as I could fit in.
Why don’t you take The FTP Challenge. It’ll tell you whether your indoor FTP is accurate or not.
Mike
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The vast majority of the differences will be due to heat. Going back many many years, there have been numerous studies done on ergometer generated power production indoors vs performance outdoors. There’s a wonderful book called “Bicycling Science” by David Gordon Wilson. One of the chapters always blew me away, entitled “Thermal effects on power production (how bicyclists keep cool”. Here are two quotes from the chapter than I think help illustrate why people (typically) see some impressive differences in their performance indoors vs outdoors:
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“For highly trained racing bicyclists attempting to pedal ergometers at a power output of 373W, a common range of endurance is five to fifteen minutes. The fact that all the racers observed were capable of outputs of nearly 373W in one-hour time trials outdoors demonstrates vividly the value of flowing air in prolonging the tolerable period of hard work.”
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“The relative air flow generated by bicycling is of such magnitude that it bears little resemblance to the drafts produced by the small electric fans often used for cooling people pedaling ergometers. As a consequence it can be said that under most conditions of level cycling, the bicyclist works under cooler conditions than does an ergometer pedaler. At high speeds, most of the rider’s power is expended in overcoming air resistance. At 9m/s (20mph) about 150W are dissipated in the air. The cooling that occurs is a direct function of this lost power. Even if cooling fans of this power level were used for ergometer experiments, the cooling effect would be much less than that of the moving bicyclist because most of the fan power is dissipated as air friction in areas other than around the subject’s body.”
Super nerdy, I know, and written in 1982 (I have the 3rd edition published in 2004), but still…it’s bears noting that discrepancies between indoor performance and outdoor performance are less about your power meter, the inertia of your trainer, and far more attributable to the lack of heat dissipation experienced inside.
Bottom line: Get an amazing fan. 
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@Oblewis, nice summary.
Joe Friel wrote about this recently with a very nice explanation: when your body seeks to cool itself, it shuttles blood to your skin. This is blood that could be used to carry oxygen to your muscles. My guess is, if you notice your skin turning pink as you work out, that’s power you’re sacrificing to cooling.
It’s all starting to make more sense…
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So based on this if I want to get faster on the road, as I’m not bothered about being faster in my shed I am better training outside. If the power I can ride at outside is 10-15% higher do I get the same training benefit (assuming I could control all the other variables) from riding at a lower power knowing that my body is also working on cooling that it won’t need to do outside
I don’t know for certain, but my intuition would say ‘no’. I think what I would do at this point is assess just how well your cooling situation is inside where you train in an effort to reduce the discrepancy between the two.
I would personally test indoors, set my FTP to what’s recommended by those tests, and focus my structured training indoors. Outside rides are great RPE tests and it’s fun to get out there and go after the local club ride or a PR on a climb, etc. But I wouldn’t test outside and then train inside with that FTP, unless I saw a much closer approximation between the two.
My 2 cents.
Oh…and I also wouldn’t be tracking TSS from a commute unless I was commuting with training intent on those rides.
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I think they’ve covered heat-related issues on the podcast. I would guess inadequate ventilation is akin to heat acclimatization, in which case you wouldn’t get the same benefits indoors as outdoors.
As for TSS, in my experience commuting can be pretty high intensity (lots of little accelerations to adjust to other traffic) and should probably be included to monitor stress on your body even if it’s not “training” per se. Besides, we’re all pretty anal retentive, aren’t we? 
I’d like to hear what your rational is for that statement.
While I don’t disagree that stress is stress and monitoring it can be an effective way to understand the training (ad overall) demands we put on ourselves week in week out, there’s a number of reasons to not track it. Obviously this is highly dependent on one’s commute. For the purposes of my own opinion on this, i view the word “commuting” as travelling by bike to./from work with the intent of not requiring a shower. I’m not training, I’m just getting myself to work or home.
So, that being said, here’s why I wouldn’t count the TSS:
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Just because I have a power meter that allows me to calculate my TSS, doesn’t mean I should. Yes, it’s “stress” when commuting by bike, but so is sitting in a difficult meeting, or having an argument with your spouse, or being worried about your kids, etc etc. You don’t have a stress “meter” for all the other non-specific training aspects of your life. So, my take is, either measure all your non-specific training stresses or none at all.
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Intent is a real thing. I’m a big fan of looking at a workout and understanding ‘why’ i’m doing it. What physiological system am i hitting? Due to the completely unpredictable aspect of most people’s commutes (again, see my definition above), it’s virtually impossible to identify a true training purpose to your commute. My logic is that it just throws my purposeful TSS tracking with a bunch of junk.
I can more easily look at how I feel week after week by judging my recovery “score” knowing I had some commutes, or some tough meetings, or a stressful week, etc. and then adjust my purposeful training plan to suit. I find this better than trying to do the mental math of “oh, right, there’s around 200-300 TSS of unstructured off/on/off/traffic/off/on commuting in my 6 weeks average…”
This is a personal thing, and simply my approach. Some people’s commutes may be great opportunities to add some purposeful training. But if you’re just waggling between traffic lights while pushing on a power meter, I am of the opinion that this is more effectively tracked in notes and recovery feelings week over week than as a component of my CTL.
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I think those are fair comments for very low intensity commutes but we were discussing the following:
That is worth tracking.
I’m actually a believer that genuine recovery rides shouldn’t have any TSS as they’re meant to be restorative rather than adding stress.
Mike
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