I’ve been shopping for a new car lately and looking at electric vehicles, so I’ve been thinking a lot about batteries and recharging options and stuff like that. And I have a question:
Why don’t electric vehicles also have a solar panel attached to the battery? So that whenever the sun comes out it recharges a bit?
Seems so obvious that there must be some technical reason why it’s not happening already.
This article talks about just that.
Earlier thread on this topic: https://boards.straightdope.com/sdmb/showthread.php?t=862649
There’s room on the roof of a typical EV for solar panels that would produce about 200 watts in bright sun. A typical EV consumes about 300 watt-hours per mile of driving.
So for each hour parked in bright sun, the solar panels will add less than 0.7 mile of range. Not worth the expense and hassle of adding solar panels.
The Prius used to have that option. It was just enough electricity to run the fan on hot sunny days.
The amount of range it would add to a modern EV would not be worth the cost or the weight.
Reading that article cited above, it sounds like it could have a chance in the future. For some. Personally, (since I park in a garage at home and under a roof at work) I’d prefer solar panels on my house.
I knew you guys would come through. Thanks!
So there’s a difference in the amount of solar energy needed to push a car, and the energy needed to power my garden path lights. Makes sense. 
Procrustus, Elon Musk would be happy to help you out with that. When I went to a Tesla showroom, they greeted me, logged me into their computer, and the 2nd question they asked was: do you own your home? Do you have solar panels? I’m sure I’ll be getting mail from Tesla’s solar panel division very soon.
It’s on my wish list.
Another reason would be that cars have a shorter expected life then solar panels (and usually solar panels take several years to pay back there cost), thus it would be better to install the solar panels where they can live their full life. Putting them on cars is sort of a waste environmentally.
One can put it in perspective without detailed numbers just by recognizing that motor power in electric cars is typically measured in hundreds of kilowatts. 200 HP is equal to 170 kW. Tesla Model S motor power ranges from several hundred to over 500 kW (in Ludicrous Mode). This is a staggering amount of electric power compared to what we’re normally used to, and certainly to the feeble power output of a solar panel. If you could hook up a Tesla Model S to power your home’s electrical system in a power outage, it would go for weeks. The Tesla Powerwall has only a fraction of the capacity of any Tesla vehicle battery and Tesla claims 7 days for an average home. Moral: it takes a tremendous amount of energy to move a car at highway speeds!
What with failing eyesight and preconceptions, I read the title as “Why don’t RV’s have solar panels too”
To which the answer is, RV’s do have solar panels. Depending on where you are going and what you want to pay. RV’s often have large roof areas, and often stop for longish periods in places where there is no available electric power, and locations where people also want lights and refrigerators and cooking.
any way… back to your usual programming.
XKCD’s guide “Should I Put Solar Panels On It?” is still pretty accurate.
It does indeed, although not as much as you suggest. hundreds of kilowatts are required for satisfactory acceleration, but not for steady-speed cruising.
One version of the Tesla Model S that I’m aware of had an 85 kW-hr battery and a highway range of about 260 miles. Assuming a cruise speed of 70 MPH, the time to cover 260 miles is 3.7 hours, resulting in an average power output of just 23 kilowatts, as opposed to the hundreds of kilowatts typically available in a passenger car.
The added weight would be an issue. And so would the aerodynamics. A car with a solar panel might end up consuming more extra energy while traveling than the panel would produce.
There are now (at least) two EV’s actually getting close to production that can recharge their propulsion battery from their built in solar cells.
So the capabilities of such a tech are starting to come into focus.
The Sono Motors Sion is a cheap (for an EV) car that can produce more than a Kilowatt peak power from its solar cells and several kWh per day, sufficient for maybe 20+ km driving range.
The Lightyear One has been actually demonstrated in pre-production form. It is an expensive (even for an EV) car that has a large (for a car) area of solar cells and a very high (for a car) energy efficiency, so promises a fairly significant solar driving range (50+ km per day) in good conditions.
That doesn’t actually prove that it’s practical, though. That just proves that the companies think they can convince enough customers that it’s practical.
Nothing is proven until these cars are in the hands of customers in sufficient numbers, but I think it’s great that things are clearly progressing to such a point.
ETA: In any case these two will both be good (Sion) and great (Lightyear) usable cars even if their solar panels don’t work out at all (if all the other features work out).
Aptera claims they have a design that allows 40 miles per day of driving just using the built-in solar panels. Though they are still trying to raise funds to build a prototype.
Dethleffs made a prototype electric RV with solar panels. That might actually make sense - park for a few days, drive up to 100 miles, repeat.
Isn’t that how Matt Damon did it in “The Martian” ?
With an RV that you’re parking for a while, you might also not be restricted to the roof area. While you’re parked, you might be able to set up more panels in the area around the vehicle (possibly doubling as awnings).
I suspect, though, that most RV customers would want significantly more than 100 miles range. What might make more sense would be to have a conventional gasoline-powered RV, that can carry some smaller electric vehicle (somewhere in the scooter to golf cart range), that charges from panels carried on or deployed from the RV, and which you can use to travel around the vacation spot where you have the large vehicle parked.
I can verify this, I have a 2010 Prius with a solar panel on the roof. It really doesn’t help that much. The biggest “benefit” would be you can turn the fan on remotely on hot days to cool down the car using the solar panel’s energy, problem is it still ends up drawing energy from the battery, so I end up with a slightly cooler car with a drained battery.
I can definitely see why they removed it from future models.