Why don't e-Cars have roof-mounted solar panels?
Assuming optimal positioning (30° tilt angle to the sun), one square meter of solar cells typically has a 100 W peak output
Modern e-Cars such as the Mercedes A Series e-Cell are rated for a power output of 36 kWh. This means that in order to fully charge the e-Cell's battery, the vehicle would have to be exposed to direct sunlight for over 360 hours. Assuming an average of 10 hours of sunlight per day, it would take 36 days to complete the charging process.