Assumption: someone crams a 300 watt solar panel onto the roof of their EV and manages to integrate it into the charging system so that it’s pretty efficient to use that power.

Numbers:

One hour of good sunshine on the 300 watt panel = 300 wstt-hours (Wh).

Average EV energy usage : 200Wh per kilometre these days.

Result:

One hour of perfect sunshine hitting the roof of your car equals 1.5 kilometres of extra range, or you can drive your car in a steady-state fashion at a few kilometres per hour.

Conclusion:

Probably better off increasing the storage capacity of the battery as a full day’s sunshine will get you about 10 kilometres of range.