in the Tour de France, how do they measure the riders' wattage ??

Used to be there were very complex, very expensive power-measuring systems that were normally only available to big teams/researchers with lots of bucks.

Now, as technology increases, even serious recreational cyclists can afford them:

https://www.fact-canada.com/Powertap/...att-meter.html

The SRM power meter: https://www.srm.de/englisch/prod_srmts_str.html

Fitted to a Dura-Ace crank:

Change that even to only.