Solar roofs on cars, why not?

So one of the biggest things that I hear people asking all the time is “Why don’t we see more electric cars coming out with solar panels on the roof? Surely that’s all free fuel just falling from the sky….”

Well, hopefully I can answer this question in an easy to understand way, and that’s not to say that it’s impossible, but that I think that a lot of people still overestimate how good current solar technology is, especially as it applies to a car.

Anyway, let’s look at some specs here, this is fairly true for most solar panels on the market, but for a specific example to allow you to check the maths, we are going to use the KICKASS 160W SemiFlex Solar Panel- Semi Permanent Caravan Mount panel, this is a fairly good quality panel, it’s not a Panasonic, but it’s certainly a good mid-range panel. There are dozens and dozens more panels out there that you can use, but we’ll use this one to get a good baseline number.

Anyway, some technical data:

Rated Maximum Power160W
Max Voltage18V
Max Current8.89A
Max System Voltage500V
Length1,500mm
Width670mm
Area‭1,005,000‬mm2

From this we can deduce some simple things, there’s a fairly large area, but there’s a fairly low output. So we can see that even if we used this one panel, for an hour, we are going to get 0.16kW out of it per hour, so it would take around 375 hours to charge a 75kW vehicle from 20% up to 100% or 281.25 hours to get you from 20% to 80% (Theoretically)

So this panel, based on those numbers, is generating 0.00015920398W/mm2, still with me? By calculating the energy per square millimetre, we can then have a good look at how much you could get when you look at the surface area of the vehicle you can put solar panels on, which for most people, would be the roof.

So taking the Tesla Model S, this has a roof area of 42 x 45 Inches, or ‭1,066.8‬ x ‭1,143‬ mm in Non-Freedom units, this gives it a roof area of ‭1,219,352.4‬mm2 which is pretty hefty, until year realise that if you covered every square millimetre of this roof with solar, you would only get a paltry ‭194.13W out of that panel. This is not really a great use of the roof area.

Then comes the next question, where do you park? Like, at home? In a Garage? No free solar in there is there, so you now have to leave your car in the driveway. How about work? A lot of people that work in cities will have underground or covered parking at their place of employment, a lot of cities are now moving to either multi-story or underground car parking as it is a more efficient use of space in these areas. So you won’t be getting any free solar in there either.

Lastly, even if you do park it in direct sun, the angle of the sun and how much sun is hitting a panel will directly influence how much power it can make at any given time.

Here is a good review based on a standard consumer solar blanket:

Hopefully this helps answer some questions on why solar roofs on cars aren’t common, but they are coming!

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