From my post in another thread, excerpted for this discussion:

Put in your effort, and your weight, get the burn rate:

https://www.bicycling.com/health-nut...ed-calculator/

Or if you don't trust it, we look here for the METS:

https://sites.google.com/site/compen...010.00104010.0

01040 10.0 bicycling, 14-15.9 mph, racing or leisure, fast, vigorous effort

So, 10.0 METS.

And use this math:
METS x 3.5 x body weight in kilograms) / 200
to get by minute, and then x 60 for calories per hour

10.0 x 3.5 x 104kg / 200 x60 = 1092

It is a guess, because you don't account for going up hills or going down. If you want Strava to be accurate, you need a power meter, and then you need to know how efficient you really are on the 20 to 25% scale.

So, either $2k in gear and metabolic testing or... guess.