Very basically, the energy captured by solar panels is…not really enough for a car. Panels will often operate at poor efficiency in many driving conditions. The panels themselves will be pretty costly and it’s also very likely that having them travel consistently at high speeds would lead to wear and tear, adding to repair costs.
Unlike other sources of “free” energy a car could make use of, like that captured by regenerative breaking, the energy of the sun hitting the car is just too expensive and cumbersome to make use of.
They don’t deliver nearly the power required. A meter squared of solar panel delivers about 1 kW under peak conditions. Let’s assume you can fit a square meter of solar panels on your car. A chevy bolt with a 66 kWh battery would take 66 hours of peak sunlight. A Tesla Model S takes 100 hours.
Meaning you’re probably losing more energy due to the added weight of the panels than you’d ever get by charging. This is why “why don’t we put solar panels on <x>” pretty much never works unless <x> is something big and static like a building.
Not enough surface area and they’re not efficient enough yet.
ELI5: Not big enough and too slow.
There’s a recent Joe Rogan Experience episode with Elon Musk and they talk about this exact concept. The only vehicle that Musk believes this could work on is a sprinter van size vehicle, think moving/delivery van. The panels would need to unfold from the roof to triple their surface area to efficiently charge the vehicle.
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