It is important that you calibrate (zero-offset) your power meter before every ride. This only takes a few seconds to do and it helps ensure that your power meter will be as accurate as possible. However, some power meters feature auto-zero. With this feature, you don’t need to worry about manually zero-offsetting your power meter before you ride. This is because power meters with auto-zero functionality automatic zero-offset the power meter for you, usually every time you coast for a few seconds during your ride. While it is still possible to perform a zero-offset with these power meters using your bike computer, it really is not necessary.
The following power meters have auto-zero functionality:
Favero
FSA
Garmin
power2max
Quarq/SRAM
Sigeyi