Turbo trainer compared to road / air resistence - Magneto Classic
This Cycleops document appears to show the speed-power curve for the Magneto trainer. A few comments: first, the actual power demanded will depend on the pressure between the tire and roller, so you will want to make sure that is consistent between trainer sessions. Second, note that the power increases roughly linearly with speed, so if you double your speed the power also doubles. This is typical for magnetic trainers, and you may want to know that in the real world on real roads, power increases roughly with the cube of speed, so if you double your speed, the power increases by a factor of eight. Lastly, there are online "trainer" services (including www.trainerroad.com) which claim to have calibrated a wide range of trainers to estimate the speed-power relationship. I have no first-hand knowledge of these services, but typically one installs an ANT+ speed sensor on the rear wheel of the bike, mount the bike onto one of the listed trainers, insert an ANT+ dongle into a nearby computer, and connect to the online service.