When connected to a solar system (based on net metering) during the day when the solar is system is generating more power than is used in the house the display will show zero. If for example you use more power than is generated then the display will only show the consumption over and above the power level generated from your solar system.
An example is that if your system generated 2kw and you were using 2.4kw in the house the display will only show 400 watts as this is the amount you will be drawing from the grid (and paying for). The reason for this is that when power is flowing back into the grid the LED light on the meter stops flashing and as a result the sensor unit will record a zero power reading.