I agree with most of the others.
Good bikes (fixed gear, magnetic resistance)
Good instructors (warm up, 20min of pyramids, 10min of sprint intervals, cool down)
Good sessions
Shit bikes (freewheel, friction resistance)
Shit instructors (no, clearly no one is going flat out for all of them if you’re asking them to do 20x 1 min ‘sprints’, and no I refuse to snap my ACL’s pretending to go round corners).
Shit sessions.
Back to back sessions with good instructors is an absolute killer if you’re fit enough.
Also I’d bet there’s a lot of heat generated at the flywheel, as the resistance is mostly friction? (could be wrong) whereas with weights etc. you’re just moving the weight and little or no heat is being generated. So That 21% number is just the heat coming off your body, I’d bet a fair percentage of the remaining watts is also getting converted into heat?
It’s all heat, the resistance unit it just converting that 21% into heat.
So yep, 20 people all putting out around 250W would be near enough a 20kW heat output (in a small room). It’s not all sensible heat though, some of it is latent heat in the form of sweat, so it’ll rapidly go from ~50% humidity to 100% (which means sweat sticks to you and takes longer to evaporate as well).