If you are like me, you are a numbers nut and having trained with powertap since 2008 on 3 different hubs, you have always been curious about the differences in various power meters. I recently switched to a crank based (the new Quarq Elsa RS for my shimano Di2 drivetrain), but still have my Powertap G3. I decided to sync up my 2 garmins (500 with quarq and Fenix3 with powertap) on a long ride to see the differences over the course of a long ride. What I first noticed throughout the day just at glancing was the constant 15-25 watt difference in the two units. (powertap showed lower). Then I thought, well maybe it's just delayed reading since it's at the end of the drive train. Maybe it's possible the quarq power readings come through 1/2-1 second faster than powertap - meaning if i surge, it takes a fraction of a second for that surge to show in the powertap vs. the instant pressure reading of the quarq just from actual distance of where it is measured.

I'm no DCRainmaker, but here is the output that I just wanted to share. I'm not saying one is wrong or one is right, I'm just saying it's very interesting data. It should be noted both were calibrated at the beginning.
note...edited later for this information which is likely the cause of the differences:
So here are the garmin settings:
Fenix - record every second, GLONASS Off, Auto Pause - On

500 - Non-Zero Averaging, Smart Recording, Auto Pause - On

So I guess the difference is smart recording on the quarq (higher readings) vs. 1 second recording on the powertap (lower readings)

Powertap high level data: (Garmin Fenix3)
Avg power - 175
Max - 643
Max avg power - 20 minutes - 195
normalized power - 184
IF .706
Work - 3463 kJ
distance 108.87 - just including these bottom ones to show the other differences.
avg speed 19.8
elevation gain 3776

Quarq high level data: (Garmin 500) not worn on wrist obviously and I find it probably doesn't have as good connection with the satellites for distance measurement since it's below my head. I say this b/c when I'm following a map, it will lose route frequently.
Avg power - 196
Max - 642
Max avg power - 20 minutes - 206
normalized power - 200
IF .768
Work - 3700 kJ
distance 108.59 - just including these bottom ones to show the other differences.
avg speed 19.8
elevation gain 3655

I think this shows at a high level where your power is truly measured from can make a difference in your readings. It doesn't mean one is more accurate than the other, it just means to trust the unit you are using and be consistent with measuring. i.e. don't train with crank based and then race with hub based or vice versa. Numbers could be drastically off and you could sabotage your race. However, if you are in my age group, feel free to test it in a race :).

This was really just a personal little science project that I always wanted to do.

Texan