It’s still worth checking the settings to make sure they are what you think they are.
But all this does indeed sound like a total mystery.
Oops, sorry.
No harm in double-checking!
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Gotcha. I’m pretty sure I did this test at the request of Favero some time ago, but I can’t remember at this point because of done so many test requests from them. I lost count some time ago. 
Anyway, the Garmin Edge 530 bike computer was connected via ANT+ and Zwift on a PC was connected via Bluetooth. The results were essentially identical for today’s ride.
Garmin:
Zwift:
I was in zone 5 for my heart rate quite a bit on this ride and was a sweaty mess afterwards:
I got destroyed on the hour long segment. I came in 244th out of 246 people. If this is accurate then I could certainly live with it, but again, at an average heart rate of 145 outdoors would yield an average power of probably 220 watts. So confusing.
What’s the cooling situation like in the room where you Zwift? Room temp/humidity? AC on? Fan preferably more than one directed at you?
The lack of adequate cooling will raise the RPE significantly for a given wattage/effort when compared to outdoors, even at endurance pace/wattages.
I’d try something really simple, like putting a pedal washer in-between the pedals and the crankarm. Or if you already have one there, then remove it.
Thanks. This was brought up earlier in this discussion. Cooling is fine and in Texas right now its freezing. This example will give you a little better understanding and why cooling likely has nothing to do with it.
If I jump straight on the spin bike and just start pedaling at 200 watts, I can last for maybe 3 - 4 minutes before my heart rate is above 175 and I have to stop. Outdoors on any bike with the same power meter pedals, I can do the same. That is, jump on the bike and start pedaling at 200 watts. I can hold that for over an hour. It is truly confusing.
There is already a washer between the pedals and the crank arm. I can’t really remove it because it is there to protect the body of the Assioma Pedals from tightening into the crank arm. Basically, it has to have a washer on the spin bike.
On one of my outdoor bikes, the washer isn’t required. On a mountain bike I have, it is. The readings on all the outdoor bikes are consistent. It is only when I move the power meter pedals to the spin bike indoors that the power meter readings drop considerably.
Any chance you can borrow a dumb trainer and try one of your bikes on it indoors with Assioma power meter pedals? I’ve ridden both spin bike and a regular bike on a dumb trainer, and the feel is completely different. I am able to produce similar results. However my brother, who is more used to riding strictly outdoors on a road or cross bike, initially struggled to produce the same watts on a spin bike inside. He does much better on a regular bike on a trainer indoors just because the feel is so different.
Thanks Lebasi. That is a good point. I have thought of the dumb trainer. I’ve also thought of one day taking the power meter pedals to somewhere that has a Kickr Bike or something similar and testing them on there. That would sort of put an end to this because if the Kickr Bike was producing wattage similar to the power meter pedals then I would know they are accurate on a few different levels.
I’ve also thought, and yes this is a little weird, but to ask a fellow cyclist (no idea who), that knows all too well what 200 watts feels like, to my home to try the pedals and spin bike themselves. If they say the spin bike at 200 watts feels like 200 watts then I would know it is me and nothing with the bike or the pedals.
All this would probably be doable and done by now if it weren’t for this dumb Pandemic. I’m still not doing group rides until I can get the vaccine and I really don’t leave the house except for solo rides and mostly essentials. Does Cheeseburger Friday count as an essential? 
Anyway, that is interesting to hear about your Brother. I could very well be in the same camp as him, but I’m still not convinced until I can get some real data behind it, which just might have to wait.
These days anything you honestly enjoy and can do safely is essential.
Yes, the pandemic does make logistics tougher. So your patience is probably a great aid. I’d bet that if you a have regular large group you ride with that possibly one person has a rarely used dumb trainer in the corner of their garage; might be worth an email or two to see. In any case, I hope you get to the bottom of the mystery soon so you can enjoy and feel confident about your winter indoor training.
Best of luck to you!
Is the spin bike made of metal? It might block RF communication between the pedals.
Yes, it is made of metal, but I would think pretty much all spin bikes are. I don’t think there is communication lost between the pedals because Favero has looked pretty deeply into the data. Not only have they received FIT files from me, but each time you zero offset / calibrate the pedals (recommended to do each time you ride, takes 30 seconds), the power meter pedals send data back to Favero in Italy. They use that data for troubleshooting. A little “Big Brother,” but I find it pretty neat.
And actually, they just released a firmware update a few days ago that requires no more calibrating / zero offset. The pedals now do all that for you right when you start riding. They only recommend manual calibration when you are switching bikes now. Nice update since it is just one less thing to think about.
Hi,
I have same type set up, spinbike + faveros and same issue. Zwift power is way too down and most interestingly i get different power reading to my Polar vantage watch, what seems correct to me.
Say i get polar 100W and the quick change to zwift gives 60W…could it be on zwift algos?
If your watch and Zwift is both paired to ANT+ then they will show the same number. Also check that both are set to updat at the same rate.
I would suggest you do a FTP test on the spin bike and use that for your training and don’t try to compare inside and outside.
You are over thinking this. HR is not a reliable source to compare.
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Hey Tony. Can you explain this a little more? How do the numbers compare after doing a full ride? What is your average power and average heart rate on both Zwift and your watch?
I respectfully disagree. Put yourself in my position. I bought a power meter for $750.00 to provide answers and assist with training, not add confusion. I have done exactly what you are describing for the past 3 months. My FTP is 174 on the spin bike. I do low heart rate work outdoors for an hour and my average watts are 210. See the problem? Both of them can’t be right. 174 is giving everything I’ve got on the spin bike and killing myself whereas 210 outdoors is pretty much relaxed. It just doesn’t make sense.
But it gets worse. Now throw in Zwift, something I really want to take full advantage of. I did Innsbruck yesterday for an hour and 20 minutes. An hour of that is one segment all uphill. I was in Zone 5 for my heart rate for the majority of the ride. Guess where I placed on the segment? 244th out of 246th. If it was clear that was accurate then I could certainly live with it. But in the real world it is a very different experience. In group rides I’m most often with the A group and pull the paceline plenty of times. My average speeds are always 20+ mph. It is a very different experience and what I believe to be an inaccuracy with the power meter pedals is taking away from my enjoyment in the virtual world, Zwift. I think a jogger even passed me once. That was humbling.
Anyway, I hope that makes some sense about why this means something to me. I do see your point though, and yes, I continue to train leveraging the power meter pedals as my tool. I just know it could be more enjoyable if it was clear they are providing accurate data.
The average watts are usually lower outdoors than indoors, because you sometimes only roll and don’t always have to pedal as you do in zwift. If the values in zwift are correct, then your values outdoors are wrong. In a large group, you don’t need much wattage outdoors and can easily roll along at 20 mph with 150 wattage. So I suspect that garmin doubles the wattage values because the device assumes a UNO. This gives very plausible numbers in your case. Many riders overestimate their wattage values in the real world, in part because Strave estimates much too high.
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Thanks. I know what you are saying, but the tests completed in the original post were done on a 2-mile loop outdoors with no traffic and no other riders (solo). This was intentional to remove the very variables you described (pedaling and not pedaling, highs and lows, drafting, etc.). It would also help mirror the spin bike the most, again, because of constant pedaling and a constant heart rate of 133 for all test rides. As you can see, based on the results and removing all these variables, the results still reflects 200+ watts for rides outdoors and around 116 watts for rides done indoors.
Also, I and Favero confirmed very early on that all the tests were done with both pedals (DUO). They can see all the data provided from both pedals and in Cycling Dynamics, I can do the same. We did take the additional testing of forcing the DUO pedals to be UNO and the same results were acquired. I even took the time outdoors and indoors to only pedal on one pedal while the pedals were in DUO configuration. The power in each case was cut in half, as it should be, since power was only being placed on one pedal instead of two.