The energy from the solar panels is converted to alternating current so that it can be delivered back to the grid and/or used to power appliances within the building. So this energy is also used to charge the car. If the energy consumption including charging the car exceeds the amount of energy generated, then no energy is supplied back because there is no surplus. This is generally the situation when the car is charging.
Why charging on just the power from the solar panels is still very difficult boils down to the fact that a car needs at least 6 amps of power to charge (per phase used), and that, even with the most optimal conditions, is very difficult with the current solar panels
A calculation to get an idea:
If you have 10 solar panels of 365 Watt peak, then with the average efficiency of 85% in the Netherlands, you generate up to about 8.5 kWh per day.
If the sun shines for an average of 10 hours, you generate about 850W of power with your solar panels.
850W is about 3.7 amps on one phase of 230V. Insufficient to charge even on one phase.