Quarq and SRAM power measurements may vary from your smart trainer or another power meter due to various factors. Including some listed below.
1) Manufacturing differences:
- Different manufacturers have different standards and claim different accuracy rates.
2) Where your power input is measured:
- Power measured from the crank arm will vary from power measured at the pedal. When measuring at the pedal, crank arms function as a lever.
- Power measured at the hub (as it often is on an indoor trainer) will measure differently than it would from the crank or pedals. Hysteresis must be taken into consideration here - the drag and flex of many components will affect the power numbers as they work through the drivetrain towards the point of measurement at the hub or trainer.
A consistent power meter is more important than the difference between power meters. Complete a zero offset calibration before every ride to ensure you get your power meter's most accurate power numbers.
To learn more about the differences in indoor vs. outdoor training, check out the "Adjusting Your Functional Threshold Power for Indoor Riding" article by Hunter Allen.