Originally Posted by
topflightpro
Typically, indoor power numbers are 10-15% lower than outdoor numbers.
Concur if the rolling resistance is lower on the trainer than it is outside. You don't get any benefit from inertia (aka real road feel). However his 20 min result was indoors so any estimate (time/duration) as a result of that should also apply indoors. In other words it wasn't an outside 20 min estimation. Personally, I skip all ramp and 20 min estimations and set my zones off my outside one hour power (deliberate TT effort, not riding around with micro recoveries). Now my trainer definitely does not have the same rolling resistance as outside, so you are right about the 10%-15% difference. It does make me work harder and I consider that a good thing.