The metering accuracy of your Zaptec Go depends on whether it has been factory-calibrated. You can check this by looking at the product label on the underside of the charger.
If the product label contains the word Calibrated, then the internal energy meter has been factory calibrated to MID Class B accuracy, which is +/- 1%. You can download the calibration declaration via the link at the bottom of this article.
If the word Calibrated is not present, then your charger has been manufactured with a metering accuracy of +/- 3%.