How is power calculated? Every app gives me wildly different results

I’m using a Kurt Kinetic Road Machine (no built in smart capabilities) and a Garmin Speed Sensor 2. I grabbed a third party app that only uses speed sensor data to measure a ride and did a quick 5 minute test that shows me at an average of 25kph for the entire ride. Probably about right for an outdoor ride in my case. No idea, don’t have much consistency outdoors.

I took a look at Kurt’s power calculation formula which seems to be the first result if you Google “kurt kinetic road machine power formula”.

Since the formula uses mph, 25kph = 15.5mph, so we get (used Wolfram Alpha, so hopefully couldn’t have messed it up):
P=(5.24482015.5) + (0.01916815.5^3)
P=153 watts

I get around the same speed in a Zwift ride (probably a bit more generous, but maybe drafting and all), but power sits at around 120 watts for what I think is about the same effort. Obviously not very scientific, especially without a cadence sensor, but a quick an easy test for an estimate in the same gear.

Also tried a few Zwift alternatives like Rouvy & Fulgaz and estimates there are around 180 watts or more. In addition, tried some outdoor calculators based on the speed sensor data and they estimate around 100-125 watts (though I had to guess a bunch of variables, so maybe useless data).

Just curious if anyone’s noticed similar issues with performance being extremely inconsistent across the various cycling apps? I expect some inaccuracy compared to direct drive, but not more than 50 watts. I would’ve assumed Kurt’s formula is accurate here since they designed it, assuming it applies to my trainer anyways.

50 watts isn’t out of the ball park IMO.

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Hmmm, in that case, might be the nudge I need toward direct drive. Accuracy might even be slightly better if I assume the Kurt formula is right, since it would put all the apps within like 25 watts. But even that seems a bit unfortunate.

It all depend on what power-speed curve the software is using.

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Although I think a direct-drive trainer is better, another option would be to get a Kinetic inRide sensor for your Road Machine. They are really cheap. With that, you can do a spin-down calibration and it transmits pretty good power data to Zwift. I have one on my trainer and it compares well with the other PM on my bike.

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For the longest time, I thought that sensor was nothing more than a speed/cadence sensor and skipped the fact that it actually transmits power as well. Thanks for this. Might be the best solution to get consistency across all platforms.

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Well, sensor arrived and totally did the trick. Averaged 158w for a 20m ride, which is right around the 153w I got from Kurt’s formula on a previous test. More importantly, I’ll still be able to track progress across various apps (though from my limited experience with the others, probably just gonna be Zwift). Even outputs a cadence (I assume from the slight variation in pedal strokes).

Don’t care at all about smart capabilities (find it annoying when I’m watching TV and the ride gets unexpectedly harder ), so probably gonna stick with this for a long while.

I used an InRide 3 for several years and feel it is the best inexpensive setup for most.
I switched to powermeter pedals this year so I can also track power outside in the Summer.
My InRide tended to be 15-20 watts lower than the pedals but the gap would tend to close as the trainer unit warmed up.I Zwift in an unheated garage so there would be wide temp fluctuations.
The Kinetic software can be glitchy.
Sometimes I would have to put my phone in Airplane mode , load the app, then turn airplane mode off.
I still used the old app.
I couldn’t get the new Kinetic app to work.
Now I just use my pedals but come Spring, I might use the InRide again for riding after dark.