I've used a stages bike and watts there is a combination of RPM/cadence and resistance. Thus, I think it's equivalent to "output" on a Peloton not resistance. I think it may be possible to figure out what watts needs to be at a certain RPM and adjust the resistance to hit that watt number. It may be a bit complicated as you'd need to know what Watts would be at different RPMs so you may have a table to refer to although maybe you'd memorize it over time. Below is an output table for Peloton from here. Also 1 Watt hour (Wh) is equal to 3.6 kilojoules (kJ).
I'm not sure if that helped but please let us know as I'm sure others have the same question!