Parked at work it will probably have a building nearby that creates a shadow. In a traffic jam, assuming perfect sun conditions and no shade, a 100W panel will generate around about 500m (or yards) of range per hour. Meanwhile the AC will use about 700W to 1kW of power to prevent your face from melting.
Some tests on YouTube report a realistic addition of 1 mile per day using the car in a typical commute.