Intervals.icu expects to find power like this:
<trkpt lat="-33.9361260" lon="18.4266640">
<ele>85.0</ele>
<time>2021-02-23T04:45:37Z</time>
<extensions>
<power>245</power>
<gpxtpx:TrackPointExtension>
<gpxtpx:atemp>23</gpxtpx:atemp>
<gpxtpx:hr>101</gpxtpx:hr>
<gpxtpx:cad>66</gpxtpx:cad>
</gpxtpx:TrackPointExtension>
</extensions>
</trkpt>
I have added support for <gpxtpx:watts>
(overrides <power>
if both present) and will deploy Tues AM (GMT+2).