That is a very good question and rollers are cool I am hoping someone else will answer. I googled it and can’t seem to figure out the best answer.
Right now I have 3-ideas.
#1 I would say if you want any degree of accuracy it would be to go buy a power meter of some sort. You would also benefit as you could compare indoor to outdoor.
#2 A cheaper option that would be less optimal is to try to determine your power by doing outdoor speed, elevation, wind and heart rate tracking to try to determine approximately your power output at a specific speed and then do the same HR indoor and find a power profile in our list on virtual power that matches that but it would be fairly involved process.
#3 Finally you could just not worry about power accuracy at all but just pick a power profile that seems slightly close based on feel compared to outdoor and take an FTP test to come up with your “Relative FTP” which ultimately will still give you accurate workouts but your FTP will just be more wrong, which is not that important other than when comparing yourself to others.
I could go into more of these options, but if you pick option #1 then problem is solved the best way.