Always plug in ..
To drive for free, use an EVSE solar adapter that is also a power diverter and always plug in when you're home. The diverter will regulate your EVSE automatically to match the extra energy produced from the sun.
When the sun is out and there is extra power, the adapter will match the power used your EV with the extra power.
If the sun is covered by a cloud, sets or another load turns on like your microwave oven, the diverter will adjust accordingly so that your EVSE will not pull any electricity from the grid.
So just always plug in when you are at home and your EV will automatically get charged up on 100% sunshine.