Solar constant is 1360 W/m². You can collect 1.3 kW (about 2 horsepowers) from 1 square meter if sun is directly overhead and conversion efficiency is 100%, no cloud, absolutely transparent atmosphere (no clouds).
Solar panels on a car can be useful, though -- to run a small fan when parked on a hot sunny day. Not much more.
#http://insideevs.com/real-world-test-2013-nissan-leaf-range-...
It's a range extension idea, not a battery replacement idea.
We're at about 15% now but let's be generous and use 20%. Neglecting the mounting angles and resulting reflection, that gives 640 watts collectible if you cover the whole car.
So you're collecting 3.2 miles of range per hour of peak sun. Note that the sun is not at peak elevation for 12 hours per day.
The middle of the country averages about 5 peak-sun-hours per day, so that means you're adding 15 miles of range per day of sitting in the sun.
It's not nothing, but it's very close to nothing. Useful to keep the car from self-discharging while it sits in long-term parking at the airport, perhaps. But it's not gonna provide meaningful range extension. The average EV has about 100-mile range, so if we assume a moving average of 33mph to make the math easy, you're moving for 3 hours, during which you collect enough power to actually move for 10 more miles. If you happen to be driving at noon in summer...
Theoretical max efficiency still uses a number of assumptions to get to 40%. If we learned anything from the path of semiconductors, it's that we shouldn't try to predict what's impossible. According to these 10-year predictions, current microprocessors are impossible to manufacture. And so were last year's, and the year before, going all the way back to the 1980s.
I'm not going to accept that infinite range cars are an impossibility. They may be impossible with today's technology, but what about the technology we have 30 years from now? Maybe solar charging is not the way we will do it, but I'm sure we will see it happen.
Or maybe not.
Electric cars still may be a good idea in a long run. Even if they can not generate their own energy.
Theoretical max efficiency of PV is about 40%, in the same way that theoretical max transistor density is one per atom. It's about the characteristics of sunlight as much as anything, and advances in manufacturing may bring us closer to the limit but not beyond it.
Theoretical max efficiency of PV also relies on assumptions about the physical properties of light in absorption and the materials being used. I'm not saying we know how to get around these limitations right now. I'm saying that we can't predict future innovations that will make the impossible possible.