ZERO OFFSET . Garmin head units call this "calibration". Each power meter has a different process (particularly regarding the position of the crank arm), so take the time to look up the manufacturer's instructions.
We recommend all of our riders do this at the start of every ride, and mid-ride if you are planning on doing important intervals or testing efforts. Note: some power meters have an "auto-zero" function, which allows you to pedal backwards to do the zero offset. We don't recommend doing this, take 20 seconds to hop off your bike and do it through your head unit.
Finally, keep a mental note of the zero-offset number displayed on the screen. This should be fairly stable between rides (within 50 units), if it changes by more than that it may be worth contacting the manufacturer.
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The little nugget of info above is from the @neroperformancecoaching instagram feed "Training Tips Tuesdays". The guys offer a weekly helpful training tip (on Tuesday's believe it or not), so give them a follow to grab some insight yourselves.