– Why do electric cars not have solar panels on rooftop so they could be charged while driving/parked
In: Engineering
They do. Most EVs have difficulty maintaining a meaningful surface to charging ratio, but the Aptera is so aerodynamic it needs less energy to cut through the air and can therefore pull real mileage out of mounted solar cells. They’re estimating over 40 miles of range a day. Check them out at /r/ApteraMotors, or at https://m.youtube.com/watch?v=bDMqEmUGxX4.
Electric cars use energy to move mass. When there’s more mass- like an extra solar panel- it means you need more energy to move the car.
The extra power the solar panel would generate would definitely help provide more energy, but not as much as the extra mass would take more energy to move. In the end, you would reduce how far the car can go.
That said, there *are* solar powered cars! They use the most advanced (and expensive) solar panels they can. Unfortunately, even those solar panels generate so little energy that the car can’t also carry big batteries- too much mass. So the car can only move when it’s sunny outside. And they carry just one person. And aren’t street legal. Also they look really ridiculous.
[https://en.wikipedia.org/wiki/Solar_car](https://en.wikipedia.org/wiki/Solar_car)
Back of an envelope calculation : good sunlight for 8 hours would allow the vehicle to travel between 5 and 10 miles. Seems worth it if the car is gonna be around for 10 years or so.
Okay, so surface area seems to be what’s limiting this from becoming standard. Being as ignorant as I am about this, I’m curious if it will be possible in the future. Have there been advancements in how much power is produced from a single solar panel of a certain size? And if so what properties made it better?
You have to drive everywhere with the weight of the solar cells, which is not insubstantial. Because cells are typically flat, they are also not shaped to provide great aerodynamics. This costs you for the materials, and measurable loss in driving efficiency.
Phoenix AZ (a **very** sunny place) averages 5.78 kilowatt hours of solar irradiance per square meter, per day. A typical car roof is 1.5 m^2, and affordable solar panels are about 20% efficient. That means, at best, you could generate 1.7kWh of power on a great day in a very sunny location.
A Tesla battery holds from 60kWh up. This means, for all that extra cost, and the losses from carrying the panels around, you might fill 1, 2 or 3% of the battery at the end of the day.
The bet is that the loss in driving efficiency and addition to the cost of the car, and that fact that the vast majority of people do no live in places like Phoenix, means it’s just not worth it.
Now, a tiny solar panel running nothing but a fan (no battery), would cost very little and may well save money on cooling. You would return to a cooler car that you did not need to run the air-conditioning as aggressively.
Current solar panels are about 15% efficient and output around 15 watts per square foot. With only 162 sq ft per parking space (~ 9ft x 18ft) maximum and less than that if only the upward facing parts of a body and if it has to be form-fitting (not a flat panel). And the sun should hit the panel exactly perpendicular otherwise rapid roll-off of efficiency and you collect even less watts. Clouds… dust… etc cut it back even more.
Anyway so at an ideal 15 * 162 = 2430 watts, and it takes ~387692307.692 watts to fully fill a Tesla S from empty, you would be parked for months or years or longer *of full sunlight* which doesn’t happen at night so that makes it take even longer. Or, get several *yards* of range from it…
It just can’t work (yet, or maybe ever).
Elon Musk has been asked this a few times, and /u/blahblahsdfsdfsdfsdf’s response is pretty much spot on: The surface area of the panel/roof is too small to make a meaningful difference. As such, it adds cost, weight, and complexity for minimal benefit.
I forget the exact numbers but it was something like ‘you’d have to leave the car out in direct sunlight for like a week to get a 2% charge’ or whatever.
In short – the surface area of a car is just too small for solar to make any sense on a car.
It would not be good while driving, but while parked it might.
The issue is that you would have to compete on cost and convenience with just installing solar panels on your garage and using the power to charge it while plugged in.
What this might help with, would be the people who still cannot even consider getting an EV, because they don’t live in a place where they can plug in their car over night to charge.
We still have a ways to go on efficiency before it is worth it though.
There *are* vehicles powered by on-board solar panels, but they’re impractical for ordinary use. I mean, look at this picture:
https://en.wikipedia.org/wiki/World_Solar_Challenge#/media/File:World_Solar_Challenge_2015-Parade_at_Victoria_Sqare_in_Adelaide,_Australia.JPG
> The 2017 Cruiser class winner, the five-seat *Stella Vie* vehicle, was able to carry an average of 3.4 occupants at an average speed of 69 km/h (43 mph).
https://en.wikipedia.org/wiki/World_Solar_Challenge
They should – I’ve seen them on top of RVs, sailboats and all kinds of other vehicles. Hell sail boats claim they’ll recharge their batteries in no time. Granted they don’t need as much as a car but when a car is parked all day at work or outside your home, it could easily be making more electricity.
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