Ok, so this afternoon I did a 20.7 mile ride in just a hair over an hour (19.3mph avg speed), and the two Garmins measured within 1 watt of each other on power. This was a ride where I tried to push hard for the entire hour, in order to get a good FTP measurement. The Edge 500 reported 218W average, the Fenix 5 reported 217W. The best 20-minute power level was 234W, so multiplying that by .95 gives 223 W, which is what Training Peaks emailed me to inform me was a new FTP value for my account. The 223 W value is what I'm going to use, because the average of 217/218 W included several minutes up front before I'd come up to speed, and that last 1/4 mile or so at the end where I slowed down approaching my house through my neighborhood. So 223 W is what I've plugged in to the apps.

I don't know why why today's ride had very close to the same power readings between the two devices. I suspect that it's because today's ride was a more steady-state affair than my other rides, with fewer times I had to pull back at an intersection. I'm guessing the differences I saw the other days since I installed the power meter were on rides where approaches to an intersection or a little coasting around sharp corners or whatever were more of a factor. My prediction is that on long rides with few interuptions to the pace and cadence the two devices will show very close to the same results, but on rides with more periods of low or inconsistent effort the slight differences in how and when they record will show up in the stats.

The 223 W FTP was slightly disappointing to me, as I'd assumed based on estimates from various programs over the years that it would be more like 240 W, but those estimates were clearly too generous. Anyhow, now I have a baseline measurement, and it will allow me to track improvements over the coming weeks or months as I ramp up my mileage and improve my endurance and efficiency.