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Comment by duskwuff | Hacker News Reader

> I think the car makers have to accelerate integrating solar charging into their cars.

Nah. Solar on cars is a dead-end -- even with perfectly efficient solar panels, a clear sky, and the sun at the zenith, the most power you can possibly get is 1.3 kW/m² (cf. [1]). I don't know exactly how large of a solar panel you could put on a car, but even with a huge 5 m² panel, you'd still be looking at over 12 hours (i.e, multiple days of charging) to charge a Tesla's 85 kWh battery. With realistic panel efficiency and sun conditions, it'd take even longer.

[1]: https://en.wikipedia.org/wiki/Solar_constant

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We can do what we do in the boating community - put a honda generator on top of it ;)

Solar panels on car is a good idea that sounds like a bad idea that sounds like a good idea.

It sounds like a good idea because if you can get your power from the sun you don't need to change.

It sounds like a bad idea because the charging rate is incredibly slow, especially compared to the rate a car can use energy. Solar power wouldn't noticeably increase the range, except for a very special-purpose non-street-legal vehicle.

However, it's actually a good idea because many people don't drive all that far in a given day. If you drive five miles to work, park in the sun, then drive back home, you might recover a pretty big chunk of that and not have to plug in quite as often or maybe not at all. The analysis is similar if you bike or take public transit to work and usually only go on medium-distance drives on the weekends.

> over 12 hours (i.e, multiple days of charging) to charge a Tesla's 85 kWh battery.

The tone is somewhat pessimistic.

Full charge every two-to-three days for a range of ... what is it? 265 miles? Effectively untethered car if you only need 50 miles daily.

Again, keep in mind my numbers are assuming a large, perfectly efficient solar panel under ideal conditions. Realistic numbers will be much worse. For example, most commercially available solar panels are 20-25% efficient.

1.3kW/m^2 * 20% efficiency = 260w/m^2

260w/m^2 * 5m^2 = 1.3kW

1.3kW * 4 hours daily sun = 5.2kW/day

265 miles range / 85kW full charge capacity = 3.1 miles/kW

5.2kW/day * 3.1 miles/kW = 16.12 miles / day

Still sounds too good, what did I miss?

That initial 1.3kW is ignoring atmospheric absorption. You lose some 300 W even at zenith without clouds. More if you're really far from the equator and/or it isn't currently solar noon. If you're in a city, you may be in shade a lot and certainly don't have the sun at its zenith for 4 hours regardless.

5m^2 is too big for a car, and the panels are heavy and expensive. Toyota used to have a prius with a solar panel roof - it produced enough energy to run the fans (not the AC) while the car was parked on a hot day. Not even in the ballpark of the energy needed to move the car (and it was an expensive option).

16.12 miles a day is not going to cut it. In your previous comment you suggested 50 miles a day. In various studies, Americans drive >30 miles a day.

16 miles would hardly even cover most American's one-way commute, let alone any other errands or trips.

>you suggested 50 miles a day

/s/suggested/calculated using data provided by parent comment/

Nobody in the entire comment chain brought up distance except for you, which you kicked off by suggesting 50 miles daily usage.

Regardless, even your drastically lower 16 miles a day isn't going to satisfy the needs of the average driver.

Affordable solar panels don't get 260W/m^2 peak. The solar panels on a car don't track the sun, so average power output is way below peak.

You don't get 1.3kW per square meter for 4 hours a day. 42,339 MW of PV installations produced 39,401 GWh of energy in 2017 in Germany[1], so on average you get peak power for just two hours a day with panels that are angled appropriately.

[1] https://en.wikipedia.org/wiki/Solar_power_in_Germany#Statist...

Then you get not all of you charge from the solar panel, but a quarter to a half. So what? That's still good. If the car basically had 20 free miles a day, that would mean much less reliance on plugging in.

You are probably going to spend more energy than that just lugging around solar panels.

Seems not pessimistic enough to me. Realistically most people would capture on average, while the sun is up, about 100 watts if they covered their car in solar panels. A car needs at least 10 KW to cruise at 60 mph. This means you need to charge for 100 hours for 1 hour of driving. Actually 200 hours because you can't charge at night-time.

I have experience with solar and boats and it is generally the same problem.

No, you won't ever be able to power a boat or a car only on solar for long distances, unfortunately. Maybe if you are driving a few miles.

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