# Why aren't there electric cars with solar cells built on top of them? [closed]

I think it would be nice if leaving the car outside on a sunny day would charge up the batteries. It would make possible long-range weekend trips to the countryside because during the 2-3 days of vacation the car would charge up.

Or wouldn't? Please help me solve this mistery. I can't come up with numbers, I'm only guessing.

I have the same question for laptops and cell phones. Wouldn't be nice if leaving them under the sun would charge them under a couple of hours?

EDIT on 20-11-2016: There are custom-built solar-power-augmented EVs in the wild:

The next generation Prius will have optional rooftop solar panel although it won't charge the main battery: http://www.autonews.com/article/20160616/OEM05/160619900/next-generation-toyota-prius-has-solar-roof-for-europe-japan

## closed as primarily opinion-based by old_timer, PeterJ, Daniel Grillo, Peter Smith, Nick Alexeev♦Apr 23 '16 at 14:50

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

• (1) As with many [naive] product design questions, the answer is "cost and benefit". A company that produces cars for general public would not add a costly feature that is useful on a long weekend trip, which occurs once a month for most people. Those who would actually benefit can add aftermarket solar panels (I've seen that done on Toyota Prius). – Nick Alexeev Apr 23 '16 at 6:04
• (2) There are laptops with solar panel on the back of the display. This one, for example. It would be more practical to have a separate solar panel that plugs into a DC jack. – Nick Alexeev Apr 23 '16 at 6:04
• technology isnt there yet. cant charge the car just on the panels on the top. there are some university races that they can do this, but the vehicle is basically panels on bicycle wheels. they do have panels now to run a fan while you are away to keep the car from heating up so that when you get back you dont burn a lot of energy running the A/C. – old_timer Apr 23 '16 at 6:21
• I'm voting to close this question as off-topic because no research was done before asking. – old_timer Apr 23 '16 at 6:24
• Dwelch, don't be a snob. It's like "i will not explain electricity because he didn't discover maxwell's equations by himself". The question is fine. – Gregory Kornblum Apr 23 '16 at 7:30

First, let's have a look at current electric cars made for everyday's use. I've choosen the Tesla Roaster, Model S and X and BMW i3:

                       Roaster  S     X     i3
Capacity [kWh]           53    85    75    18.8
Range [km]              350   500   381     190
Mileage [km/kWh]        6.6   5.9   5.1    10.1


The BMW is trimmed to high efficiency, while the Teslas are larger and are a bit more racy. And keep in mind, this is official data. If this is as realistic as mileage data for petrol driven cars, then... well...

Sunlight at the equator gives us about 1000W/m², but the efficiency of available solar cells is somewhere below 30%. So you get just about 300W/m². Next, the sunlight has to hit the solar cell perpendicularly to get the max. power. But if you place the cells flat on the floor / car, they generate just a fraction of their max power most of the day... In addition, weather reduces the power.

I've found a website which takes meteorological data into account to calculate the monthly energy output of solar cells. The site is german (you can switch to english, but I didn't find this calculator on the english version). It takes orientation (both 0 for flat on the floor), peak power in kWh (0.3) and position (click on map), and I got this min/max values for winter/summer. Daily data calculated by division by 30:

             Monthly [kWh]  Daily [kWh]
California:      25 - 60    0.83 - 2.00
New York:        15 - 43    0.50 - 1.43
Germany:          8 - 48    0.26 - 1.60


The most economical BMW i3 makes between 2.6km (1.6 miles) in german winter to 20.2km (12.6miles) in californian summer per daily charge from 1m². I quess this car can not have more than 3m² of solar cells.

And keep in mind: This are average values. What if one or more months are unusual dark? And never park in a parking garage, in shadows of trees or buildings, ...

So, today, solar cells don't give enough energy per day for a car. But they would cost a lot, while a full charge from the wall outlet is quite cheap.

• Thanks for the answer! Actually my reading of this is quite the opposit. In avarage we can say you can get 10 km per day for free. That means if you live in a 5km radius of your workplace you wont have to charge if you can park outside. Or if you live in a 10km radius, you can double the mileage of the car. That's impressive. – Nandor Apr 23 '16 at 17:32
• To get ideal charging, you would have to find somewhere that is in the sun all day. No trees or buildings casting shade. That's not likely to happen in the city unless you have a rooftop car park. – Simon B Apr 23 '16 at 22:37
• This is a great answer. However, there is a fairly major practical problem involved in converting the surface area of the car into solar panels. There are flexible solar panels available, but they are even less efficient than the rigid ones. Rigid ones cannot be readily integrated into the surface of stylish and aerodynamic cars. – mkeith Apr 24 '16 at 4:21
• They could if they use aerodynamically shaped glass above the panels. – Nandor Apr 24 '16 at 23:14
• Thinking further this topic, there's a problem. If the user of a solar-augmented EV could park in the shadow but chooses the sunlit area for free energy then the car will heat up and it takes a lot of energy to cool it down when leaving. Although it might be a positive outcome game even in this situation. And if the user can wait (works) until the sun goes down and the weather cools then it's not a problem at all. – Nandor Nov 20 '16 at 20:35

In order to charge a car in any reasonable time frame, a very large solar array would be needed. Much larger than the car. Also, existing high-efficiency solar panels cannot be molded into sleek shapes that would allow a car to be aerodynamic. On a typical car you would be able to get less than 1kW of solar panels. It would take weeks to recharge the battery at that rate.

I do think it would be a good idea for utility vehicles on larger estates if they are not driven continuously. You could just drive it around all the time and not worry about parking it at a charger. Also, their batteries are a lot smaller than passenger cars, and they are not aerodynamic. Often they have a rectangular roof canopy that would be perfect for solar panels.

Solar power at full sun is ~~ 1000 Watts/m^2. Nest efficiency going is around 30%.
Most are rather lower.

1000 Watt x 30 % = 300 Watts/m^2 or 0.3 kWh per hour of full sun per m^2.

Full sun equivalent ours/day are lower than most people expect.
Typically 2 or 3 SSH per day in winter and around 6/day in many places.
Kabul is good at ~ 8 SSH/day midsummer, but then, you'd have to live there.

If you had say 3 m^2 of PV panels on a car - rather hard, and 6 SSH and 30% efficiency - all on rather enthusiastic side.
Energy/day ~= 3 m^2 x 300 W/m^2 x 6 hours = 5.4 kWh/day
IF you could store this t 100% efficiency (you cant) and apply it to the wheels at 100% efficiency (you cant) then you'd get about
10 kW x 1/2 hour OR
20 kW x 1/4 hour.

20 kW may be OK for town driving.
So you may get 10-15 km on flat if all the lights were green. Depends on size of car etc.

Useful but not stunning.
In winter and cloudy days etc .... :-(

• I think it's very useful. And it can be enough for a bunch of commuters. Or people who use their cars only once or twice a week. – Nandor Apr 23 '16 at 17:40
• @Nandor Canbe - but note that my answer was upper end of likely - mid summer, very high efficiency cells, perfect storage etc. Real world is more in the few to maybe 5 km per day. Overall its better than no energy at all BUT often paying the grid costs or petrol cost is just easier and cheaper overall. Batteries are costly and heavy. 3 m^2 of panels wreck your cars looks and shape and etc. – Russell McMahon Apr 24 '16 at 22:15