Power meter reading difference – how to reconcile best between bike's power meter and smart trainer?

Got a wahoo Kickr. It displays 215W for a given effort.
Got a Rotor Inpower on my bike and it displays 200W for that same effort.
I sometimes ‘borrow’ my wife’s MTB on the Kickr. Has no power meter.

Power Match drives the Kickr based on the Rotor power measurement. This provides training consistency if I use my bike on the field and on the trainer.

If I got an FTP test using only the Kickr as power reference (ignoring the Rotor’s reading), I know I get consistent training regardless of using my wife’s or mine on the Kickr; but
for outside workouts on my bike with the ROTOR, I would I need to adjust my session to -15W, or else I’d be training a different energy system than intended by Trainerroad’s Plan Builder based on the Kickr power output reading.

The 15W delta impacts quite a bit on the (on/post) workout nutrition. Which one should I use as a refrence to refuel?
Is the 15W difference between both meters ‘linear’? How to check for that?

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Welcome to the club:
https://www.trainerroad.com/forum/search?q=power%20meter%20trainer%20difference%20%23equipment

No simple answers, but you can likely bet that the differences are NOT linear. Meaning the delta (in percentage) is not the same at lower power vs higher power.

Two of the best in business have some reference info:

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bloody hell.

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thanks so much for sharing (the pain) ; )

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Just pick one to use (I recommend the rotor so you have it both in and outdoors) and compare apples to apples. If you’re using the same equipment, even if the numbers are our of spec, you’re still consistent with yourself and the only real downside is comparing numbers to others. While there is merit to comparing numbers to other riders, I always find it considerably more valuable to compare myself to myself when it comes to training. If we’re talking Zwift, however, that’s a whole other can of worms.

The easy solution is just to use your Rotor power meter inside and out. Record all power from a single source. I have the same issue and just take all power from my Stages even though I’d get a better vanity FTP from my Kickr.

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Use RPE and HR with resistance mode instead of erg mode for indoor rides on the wife’s bike. Since the fit probably isn’t perfect either, don’t do key workouts on a borrowed bike and go through the trouble of switching out the bike and use powermatch

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Get the wife a PM :joy:

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Don’t do that. Use PowerMatch and then you have same power meter for inside and outside.

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On the linear vs non-linear front, I wouldn’t assume that the difference is linear.

I recently installed a 4iiii power meter and went through the same process of trying to compare it to the readings from my trainer (Elite Suito) to see if the difference was static or varied with power. I ended up finding that the biggest factor affecting the difference was actually TIME!
The further into the workout, the lower my 4iiii power meter reads compared to the Suito (when the trainer was controlling the resistance in ERG mode). They start out within a few percent and it gradually grows to 8 or 9%, regardless of the resistance/power. Assuming the 4iiii is more accurate I guess this means that the last intervals in my workouts have been easier than the first ones.

Next ramp test I’m switching to just relying on the 4iiii and PowerMatch!

I agree that zwift is the thing that makes it complicated. Because for general riding, it doesn’t matter, right? What matters is who crosses the line first, not who had the highest power. But with zwift, power IS your performance!

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Is the 4iiii single-sided? If so, it could mean that as the ride progresses your L/R asymmetry grows.

The Rotor Inpower is a left-side only power meter. Most riders’ L/R balance varies with cadence, with pedal force, and with fatigue so it’s not common for observed power to vary linearly with load; it’s more common for observed power to vary nonlinearly.

But that doesn’t address your question. There are ways to determine whether your Inpower is more accurate than the Kickr, or vice versa, but they’re a pain in the butt so hardly anyone ever does them. And there’s no simple way (because of the nonlinearity) to adjust the Inpower or Kickr readings so they match. The good news is that, to a reasonable degree, most training plans don’t really require spot-on precision in following the ride – you’ll get most of the benefits even if you’re off by a little. It’s not like a zero-one problem, it’s more of a graduated spectrum. Nutrition and refueling sort of work the same way – you don’t usually know your true metabolic efficiency so we use a rule of thumb that kJ of work is roughly equal to Calories of food, but that’s just a rule of thumb. You’ll figure out over time whether your refueling strategy is undershooting, overshooting, or about right. The one place where the zero-one may show up is if you’re racing in Zwift, in which case you should use the Kickr since it reads higher.

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Yep, same thing with me…my garmin v3’s are 30w lower than my trainer (saris h2).

I now use my v3’s for both indoor and outdoor, so it’s consistent. Certainly a rude shock and ego check when I realised I had to drop my FTP by 30w.

It’s basically Segal’s law: A man with a watch knows what time it is. A man with two watches is never sure.

Pick one and go with it. Picking the one that you can use inside and out makes sense to me.

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I’m in a similar situation. My smart trainer reads ~20w more than my crank power meter. Power for me on the indoor trainer is lower than outside.
My go to is to just use the smart trainer power and then use the cranks outside, makes it for me closer to actual, rather than having to have two different ftps or power targets. Also once I wrecked my old bike and kept it as a trainer bike, it means I don’t need to worry about swapping the pm over and I’m happy that 300w on one bike is pretty close to 300w on the other.

Good thought about the asymmetry, but the I actually have the dual sided version.
I even tried doing a second spin down calibration on the trainer before the last interval in a session and that decreased the difference between the two. My best guess is that the resistance on the trainer keeps changing as it heats up over the course of the workout.

Is the right side mounted on a Shimano crank?

Haha. I could even do it in secret.

Thank you all for the awesome comments.

This my first post to gain traction.

Plus, this morning I started (manually) taking data to (eventually) graph the delta in power at different rpm. Just for the bro science.

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