Offset is a value that is applied to essentially re-zero a power meter when there is no load being applied to it. This is similar to using the tare function on a scale to reset the zero point. This helps account for variances due to changes in environmental conditions and is a good idea to do prior to each ride.
All of our current power meters use zero (0) as the starting point for offset ranges. The unit of measure we use across all platforms is Newton meters (N m). This is true of both ANT+ and Bluetooth (BLE) communication protocols. The only difference is that for BLE it is N m/10.
For more detailed information about PowerTap’s offset values, refer to this blog post.