Originally Posted by
wgscott
Power = work/time I am going to guess both Garmins treat the numerator the same and that the difference arises in the denominator, i.e., different moving times. That should be something you can check -- did the two record different moving times when they reported different power outputs? The last ride serves as a positive control -- I am assuming from your description that you didn't stop midway, or did something different in that last ride with respect to moving times.
I think you probably nailed it. I'm betting it was different moving times. On ride 1 the Edge 500 was set to auto-pause, the Fenix 5 wasn't, hence the 15W difference due to different moving times. The course I was riding on crosses multiple streets, and whether I stop for a few seconds or just stop-and-go (or, gasp, just kind of slow-roll through) is going to be a little different from ride to ride. On rides 2 and 3 I had set the Fenix 5 to auto-pause, but it still pauses and restarts a little differently than the Edge 500, so although there will still be differences in moving times, they would be smaller, hence why rides 2 and 3 showed smaller differences of like 7 or 8 watts between the two Garmins' output. Ride 4 (yesterday's ride) I managed to avoid anything more than instantaneous stops, and as I did my laps through this one neighborhood's ring road I managed to avoid having to fully stop at any of the intersections, so whatever moving time differences there were would have been even smaller, hence the 1W difference.
To confirm this I'll turn off auto-pause on both units. They should be within a second of each other, because I start and stop both units within about a second of each other. If that was the reason for the differing results, they should show either the exact same power results, or at most within 1W of each other. My ride today will be deliberately slower than yesterday's ride, because yesterday's ride was right up against my threshold heart rate for nearly the entire ride, on purpose, for FTP measuring purposes. I'll take it easy today so I recover enough that I can ride again tomorrow and Saturday as well, then take Sunday off. With auto-pause off on both units I should know by this afternoon if time differences were the explanation for what I've seen.
Btw, I'm thinking seriously about ordering a Garmin 530. While the Edge 500 has served me extremely well since 2014, I'm really, really loving the features my Fenix 5 watch has, including constant connectivity with my phone, instant auto uploads of completed activities, etc. The Edge 530 has all those same features, plus a larger and easier to read display. The reason I've been comparing the Fenix 5 with the Edge 500 for my bike rides is mainly due to the ease of use benefits of the Fenix 5. If I pull the trigger on the Edge 530 I'll have the best of both worlds.2