Zwift does it, using the known power curves for various bike trainers. The only (small) variable here is how much power is lost with tire flex. Otherwise, it's strictly how fast the trainer is spinning, measured using rear wheel revolutions per minute. (I've seen mentions that the fluid resistance changes a little as it warms up, though.) A trainer has no wind resistance or gravity effects at all, and those are huge when riding outside.

For instance, Kurt has this power vs speed chart. 400 watts at 25 mph, for example.