Iād start with a detailed breakdown of your cooling setup.
From my experience, a vast portion of indoor riders are significantly under fanned.
Obviously, a lot depends on your indoor riding conditions. Temperature of room, humidity etc.
To put it into perspective, at just 200w you are producing between 800 and 1200w of heat. The entire time. You are basically a human shaped space heater. At higher power, the heat generated is almost comical.
Without adequate air movement, any sustained power will be heat limited indoors. RPE will be higher etc etc.
To give you an idea, I have 3 powerful fans, positioned perfectly. If my garage if temperature goes beyond just 20C (68F) I am not able to cool myself adequately much beyond 200w. Any higher in power and sweat will not totally evaporate and will eventually glisten, drip etc. When sweat drips, itās by definition, wasted energy. Youāre body is failing to cool itself appropriately.
In comparison, outdoors at the same air temp, I am able to ride at the same power output, with zero visible sweat.
Because, at around 34kph (21mph) the cooling provided from air movement is vastly more powerful.
Iād bet, you are under fanned. Almost everybody is, outside of winter.
Personally, I have my indoor FTP set 10w lower and this generally works well. Additionally, I generally, only train Z2 / Z3 indoors. The opposite of how most indoor riders train. Most indoor riders are doing very regular high power intervals, where cooling, bike movement, sensory input are all sub optimal.
Result, you are likely able to produce less power.
Not always, with practice and an ideal air conditioned training room, you can get very close to parity.