Power meters and smart trainers measure power in different ways, so it is natural that the power reported from the two sources will differ. Pedal-based power meters tend to be the most accurate, however, the high-end smart trainers now provide power readings that are +/-1%. Here are some tips and tricks to make sure that they match up as best as possible. Please keep in mind that there may still be a gap.
- Power measured by a direct drive smart trainer may be affected by losses from the drivetrain. A clean chain and clean gears can help minimize (but not eliminate) this loss.
- Be sure to perform a spindown calibration on your trainer using the trainer's native application. Then be sure to calibrate or do a zero offset on the power meter.
- Power measured for a wheel-on smart trainer will be affected by rolling resistance, tire pressure, temperature, and calibration. For best results be sure to do a proper warm-up for yourself and your trainer prior to calibration.
What about indoor numbers and outdoor numbers? There's an article here that will help explain what's going on: Ask the Experts: Indoor Cycling Power vs. Outdoor Cycling Power.