[MCRed]

When I was a kid, I saw the movie Logans Run (1970s). They made it to washington DC and found a car. They didn't know what it was, but they shined a light on it, and it started to activate and so they conclduded "maybe this thing is powered by the sun"... and it was.

Well, it's been 40 years since that movie was made. Alas, a solar powered car may still be many years off... but the elegance of it is certainly extremely appealing.

One problem: I love the Tesla's expansive sunroof. Would want solar collectors that are at least semi-transparent to light.

[derekp7]

My back-of-the-envelope calculation, if covering an entire car with solar panels... Assumptions: car width is 5 ft, hood is 5 ft, roof is 3 ft, and trunk is 4 ft makes 60 square feet for the whole car. (Note, this isn't the dimensions from a Tesla, but from the older cars that I remember). High output solar panels are 15 watts per sq ft. So 900 watts output max (lets round it off to 1 kilowatt to make the math easy).

Now taking the rated specs from a Model S, a 60 kWh battery gets you 200 miles. So .3 kWh per mile. Therefore, leaving you car to recharge in the sun for 8 hours while you are at work, will get you home if you live within 25 miles. Assuming maximum efficiency of course. Note, that you can't put 60 sq feet of solar panels on the Model S, due to the long slope of the front and back windows, and redesigning it so that it has the non-window surface areas of my old 85 Chevy would make it less aerodynamic. But this gives you a good upper bound if I did my math right.

[mikeash]

There just isn't enough surface area. A Model S has a roof area of around 9m^2. The sun provides about 1000W/m^2 on a clear day at high noon when directly overhead, so with 100% efficient solar cells on a perfect day, you'd get around 9kW of electricity. A Model S goes about 4 miles per kWh when it's doing well, so in this ideal situation you'd top out at around 36MPH. In the real world, the sun isn't directly overhead, and real solar cells have more like 20% efficiency, so cut that number down by a factor of ten or more.

The best way to build a solar-powered car would be to have a battery-powered car that gets recharged from solar energy. Basically, a Model S and a home solar generating system.

[baddox]

That illustrates the advantage of electric cars, even if the electricity is currently generated by fossil fuels.

[ctdonath]

I recently ballparked the cost of a home solar charger for my Leaf EV. Came out around half the price of the car ... not cheap, but feasible for someone serious about it.

I do have a propane-powered electrical generator for emergency/backup home power. Some day I'll try charging the Leaf with it.

[mikeash]

Have you ever worked out your approximate cost per kWh for the generator? I imagine it's painful compared to what you get from the power company, but I'd be curious to know just how painful.

[ctdonath]

About $1.23/kWh.

Ad verbiage for the allegedly 3250 running watts generator states "engine run time of 10 hours at 50% on a common 20Lb (gas grill type) cylinder". So that's under $2 for 1.625kWh, which is roughly the power draw for recharging the Nissan Leaf in question, which takes 20 hours - costing upwards of $40. More to your point, that's about $1.23/kWh. Not cheap, but a 10x markup is appropriate and acceptable for emergency needs.

[mikeash]

Thanks. That sounds pretty reasonable for a generator, considering.

[SupremumLimit]

You probably mean 0.9m^2. 9m^2 is a decent size bedroom.

[mikeash]

I shouldn't have said "roof," I meant the whole vertical-facing area of the car.

[mark-r]

Unfortunately that was only science fiction. I don't think the sunlight that lands on a car would be sufficient by itself to charge the system to any acceptable level.