Power Accuracy
Power accuracy of both the trainer and third party power meters can be affected by a variety of factors. The trainer's power readings are calibrated during the auto-spindown calibration process.
Before proceeding, be aware that no two power meters give exactly the same reading. Multiple identical, simultaneously broadcasting power meters have been empirically proven to report different power numbers, even during the same workout.
Factors responsible for this difference include:
- Freedom of motion
- Data transmission rate
- Packet loss
- Wireless interference
- Sensor or strain gauge location*
- Temperature
- Humidity
- Differences during calibration
- Battery life
- Accuracy tolerances
- Leg imbalances
* Specifically because of their location -- pedal and crank-based power meters will generally read
- marginally higher than a hub-based power meter in all scenarios
- substantively higher than hub-based power meters in instantaneous power
as all power is mediated through the drivetrain. The dirtier or more worn the drivetrain, the greater the power loss between power input at pedal/crank and output at wheel hub. Note, this power recording difference is because of precision and accuracy in both devices measurements.
Additionally, trainer power accuracy tolerances of +/- 1-2% combine with your other power meter's tolerance. This means that if your your third party power meter is rated to report +/- 3%, your total combined accuracy tolerance will be +/- 4-5% at any given time.
Lastly, be aware that it is also easier to generate power outdoors than indoors, as explained in this article on the Wahoo blog.