> - 200W would allow for 2-4miles on the most efficient cars
Sorry, but that's wrong. They meant 2-4 miles per 1 kWh, not per 200 Wh.
Very good EVs get about 4-4.5 miles / kWh at optimal speed (usually between 25 and 35 mi/hr). So, best case, 200 Wh is good for about 1 mile, assuming you can deliver the 200 Wh over a period of 2 minutes.
However, 200 W can't push the car at 1 mi/hr because of "vampire" losses: Power for accessories and vehicle systems (like power-assisted steering, power-assisted friction brakes, the ABS controller, the airbag monitor, etc). 200 W of power isn't just slower than walking: In a normal-sized car, you will literally go 0 mi/hr.
More importantly, 200 W/m^2 is the best-case scenario at noon, in the tropics, on a cloudless day, with no shadows, using expensive, high-efficiency panels aimed squarely at the sun. As soon as any of those qualifiers is not met, power drops precipitously.
In particular, the atmosphere absorbs a large amount of light: In space, solar insolation is about 1300 W/m^2. At earth's surface in the tropics, solar insolation is about 1000 W/m^2 at noon. At higher latitudes, or other times of the day, solar insolation is lower.
A good rule of thumb is that (for Europe and North America) the total insolation over a full day is about 5000 Wh/m^2. So, a solar panel on a sun-following mount is limited to about 1000 Wh/m^2/day, again assuming no clouds. For a fixed solar panel, such as on the hood of a car, you'll get about half of that: 500 Wh/m^2/day. Remember, you need 1000 Wh to go 4-5 miles.
That "4-5" implies a lot of accuracy, but I would have thought that the distance you can travel on 1000 Wh would vary by orders of magnitude depending on the speed, terrain and road surface.
However, if you are moving a 1 tonne vehicle up a 5% gradient then you won't do more than 4.5 miles on 1000 Wh, if I've calculated that correctly.
> What about this particular instance of them being wrong annoys you so?
While I can't speak for the person to whom you're replying, I can tell you why it is annoying to me, and that is very simple. In this thread there are basically two kinds of posts:
1. Posts about the physics of why solar on cars is basically a bad idea (or at least a massive waste of money in the face of much better alternatives).
2. "Yes, but have you tried turning your head sideways and squinting at it like this [...]"
For some reason, solar-on-cars brings out the #2 in many otherwise smart people, and it's extremely tiresome to constantly slap down the nonsense. Because, I think, they really, really, really want to believe it can work.
> People are wrong about stuff all the time. Often more committed to worse ideas than this.
Yes, but when you show them why those ideas can't work, using numbers and science, they abandon those ideas.
Sorry, but that's wrong. They meant 2-4 miles per 1 kWh, not per 200 Wh.
Very good EVs get about 4-4.5 miles / kWh at optimal speed (usually between 25 and 35 mi/hr). So, best case, 200 Wh is good for about 1 mile, assuming you can deliver the 200 Wh over a period of 2 minutes.
However, 200 W can't push the car at 1 mi/hr because of "vampire" losses: Power for accessories and vehicle systems (like power-assisted steering, power-assisted friction brakes, the ABS controller, the airbag monitor, etc). 200 W of power isn't just slower than walking: In a normal-sized car, you will literally go 0 mi/hr.
More importantly, 200 W/m^2 is the best-case scenario at noon, in the tropics, on a cloudless day, with no shadows, using expensive, high-efficiency panels aimed squarely at the sun. As soon as any of those qualifiers is not met, power drops precipitously.
In particular, the atmosphere absorbs a large amount of light: In space, solar insolation is about 1300 W/m^2. At earth's surface in the tropics, solar insolation is about 1000 W/m^2 at noon. At higher latitudes, or other times of the day, solar insolation is lower.
A good rule of thumb is that (for Europe and North America) the total insolation over a full day is about 5000 Wh/m^2. So, a solar panel on a sun-following mount is limited to about 1000 Wh/m^2/day, again assuming no clouds. For a fixed solar panel, such as on the hood of a car, you'll get about half of that: 500 Wh/m^2/day. Remember, you need 1000 Wh to go 4-5 miles.