[janosch_123]

I built my own electric cars and calculated if this would be worth it. Roof of car is curved and you get the conversion losses (needs to be bumped to 400V to charge batteries).

You add a lot of complexity for marginal gains. Peak time you get maybe 500W which doesn't go very far.

I haven't made video about solar yet, but I am sharing what I know on https://www.youtube.com/@foxev-content

[pchew]

I have a 100w solar panel on top of my car...to tend a 12v battery. It's got a Dewalt battery charger, mikrotik ltap, and raspberry pi hooked up to it. Little hotspot with multiple sims and resource server(mainly just for fun). Anyone that can do basic math should immediately realize there's just not enough area to make an appreciable difference in regards to mileage.

[barnas2]

The Prius Prime solar panel roof I think can net 3-6 miles a day under ideal conditions (which we're probably close to here in Arizona). I think that's a little more than people would expect, but still only applicable in niche conditions (tiny daily commute, or a longer non-daily commute). I think the math works out to ~4-6 years to break even for the cost of adding the solar roof assuming $0.15 per kwh, which isn't terrible.

If solar tech gets more efficient or cheaper, I think it starts becoming a much more attractive option in some areas. If you get into the 10+ miles per day range, that would cover a lot of peoples commutes in certain areas.

[thegrim33]

13.6 kWh battery. 39mile EPA range. Equals 2.87 miles of range per kWh. Leaving it out for 8 hours straight, on a sunny day, in LA, netted 915 Wh. Or, 2.86 miles. [0] Not 3-6, 2.86.

2.86 miles of charge, but only if left outside, uncovered, in full sun, on a fully sunny day, for a full 8 hours, in a place that gets effectively the maximum amount of solar radiation per day out of anywhere in the entire country.

Now, do the same experiment anywhere else in the country, that doesn't get max solar radiation, or that can't get full sunlight for full 8 hours, or where it's cloudy at all, or rainy at all.

2.86 miles per day is the practical MAXIMUM, given perfect conditions. For the average scenario it'd be some fraction of that.

The 6 miles figure is what they said you'd get if, in addition to perfect conditions, "if the sun shifted its orbit" (?) and gave perfect sunlight for 12 hours straight. Which is a number which should obviously not be thrown around as if it's obtainable.

The fact that they're quoting numbers about what range you'd get if the solar system was constructed differently also makes me doubt the impartiality of their experiment and the numbers they provided.

[0] https://www.motortrend.com/features/the-2023-toyota-prius-pr...

[cornholio]

> 2.86 miles per day is the practical MAXIMUM, given perfect conditions

In your particular setup.

A typical car can expose about 3 square meters of lateral area for those same 8 hours, and receive 3 kW of irradiance. multijunction cells can exceed 50% efficiency, so we're talking about a theoretical upper limit of 12 kWh electric per day.

That would require a vehicle totally covered in cells, including the windows, so not very practical, but adding up to 30 miles/50 km per day is nothing to sneeze at.

We could also imagine all sorts of solar receivers that engage during parking and inflate the apparent surface within the limits available, track the sun etc. to maximize energy.

[labcomputer]

This is all wrong.

> multijunction cells can exceed 50% efficiency

The maximum demonstrated efficiency of a multijunction cell, in a lab, WITH CONCENTRATION is less than 50%. Commercially available cells are lower.

Concentration is an important caveat for two reasons:

First, it implies that you are collecting light from a larger area than the PV panel itself. Second, efficiency grows with increased irradiance (so efficiency will be lower without concentration).

> 3 square meters of lateral area

Lateral area is meaningless. It’s all about area perpendicular to the solar axis. Unless you are driving a box van or a big pickup truck, there is zero probability that you can put 3 kW of irradiance on your panels. Neither of those vehicles will achieve kWh/mile numbers anywhere close to a Prius.

In practice, you need to halve the efficiency and more than halve the collection area you quoted. You also need to account for conversion losses.

[cornholio]

This is a theoretical exploration of the hard limits, not an engineering design.

The multijunction theoretical efficiency limit is 87% with infinite junctions, and over 50% with a practical number of junctions. There's nothing stopping you from creating a miniatural concentrating solar device that focuses the light from a 10 cm^2 area onto a 0.5cm^2 cell, we haven't seen such devices because the cost and extra mass exceed what you get from the efficiency gains when you can simply increase area; a very area constrained application with high power requirements might change that.

> It’s all about area perpendicular to the solar axis. Unless you are driving a box van or a big pickup truck

Again, what stops the top hood and engine cover of a Prius from raising at an angle and tracking the sun, perhaps even unfurl additional area? what about the area of the doors and windows?

Current solar cars can drive 1000 km per day with an average speed approaching 100km/h. It doesn't seem completely out of the realm of the possible to achieve 50km in an hour for a passenger car that can expose similar area while parked.

[conjecTech]

EPA range tends to be pessimistic for EVs as it assumes you are always traveling at highway speeds. Even small reductions in speeds can make EVs much more efficient since drag is quadratic. A quick google search shows Prius prime owners reporting 4-5.5 miles/kwh, so the 3-6 mile range is entirely plausible.

[bangaladore]

> EPA range tends to be pessimistic for EVs as it assumes you are always traveling at highway speeds.

EV EPA range historically has been overstated. However, the water is muddied because the EPA doesn't really force the manufacturers to give an accurate number. A manufacturer can choose a highway only test, but then also arbitrarily decide to derate the value (EPA example is 70%). A manufacturer can choose to include city driving in the rating and weigh it accordingly and also derate the value (if they want).

Tesla traditionally (still the vast majority of new and used EV market share) has been the only manufacturer that uses the highway + city driving tests. People then get surprised when the car cannot do the full range at 85 MPH.

All in all, this is the EPAs fault. For EVs they really need two numbers, city driving range and highway driving range. EVs are so much more efficient than ICE that speed makes a huge difference given there substantially smaller energy density.

https://www.epa.gov/greenvehicles/fuel-economy-and-ev-range-...

[slaw]

EPA's city cycle speed is an average 19.6 mph, highway speed is an average of 48.3 mph. City range weighted at 55% and the highway range at 45%.

[bangaladore]

I don't think it's that simple. IIRC you don't have to do the city testing.

https://www.epa.gov/greenvehicles/fuel-economy-and-ev-range-...

[pchew]

Everyone is also glossing over the distinction that regardless of the actual amount, it's not at an actual voltage that can charge the battery to add mileage. You can hypothetically say that because it's offsetting the power usage from the AC that it could theoretically be saving that battery usage...but there's so many gross assumptions being made that it's a pointless statement to make, and it's all out the window the second the car starts the ICE side of the hybrid drive system for even an instant.

[Dylan16807]

> Everyone is also glossing over the distinction that regardless of the actual amount, it's not at an actual voltage that can charge the battery to add mileage.

Neither is the voltage when you plug it in at home. The car has a unit specifically to convert the voltage.

If you're saying they didn't connect the right wires for that, that sucks but is easily fixed.

> it's all out the window the second the car starts the ICE side of the hybrid drive system for even an instant

Nah, doing a drive where it's 99% solar power and 1% "burned an ounce of gas to maintain the engine for the month" is fine.

[Dylan16807]

That article sure has 49 pictures, none of which show more than the very edge of the solar panel.

But looking at some proper pictures, it covers most of the midroof with 7x8 tiles of solar. But you could fit a good percentage, even sticking with a design where a huge amount of the roof into the trunk is all glass. And there are no panels on the hood. So that's an easy doubling right there, more with an average car roof shape.

[schiffern]

  >13.6 kWh battery. 39mile EPA range... 915 Wh... 2.86 miles, not 3-6

If we're picking nits, the usable capacity is only 10.5 kWh (11.5 kWh with AC-to-DC losses), so it should be 3.40 miles. Not 2.86, or even 2.62 (= 39/13.6 * 0.915).

[bgnn]

I wonder how much extra range you would get if one leaves the car in the shade so that it doesn't get super hot and there is no need to turn on the AC hard? I bet it's more than 2.86 miles.

I believe having a carport and house roof covered with solar panels + (PH)EV is the best option.

[beAbU]

Does the extra 3-6 miles factor in the need to now run the AC much more aggressively because the car will be hot from sitting in the sun all day?

If this quoted number comes from the manufacturer itself, then I think the answer is "no".

[thinkalone]

The initial use of solar on the Prius was to power a ventilation fan while the car was parked, and the current version seems to specifically be designed to provide power to the air conditioner while driving. But, I also can't imagine the difference between cooling down the cabin is much different from parking in the sun or in the shade - you'd be running it continually to achieve "room temperature" during the entire drive either way.

[saltcured]

You can't imagine that air conditioning power draw varies with the heat load that it is working against? As a heat-pump, it takes more energy to move more energy.

In the old days, they used duty cycle to adapt to the changing load. Modern ones do things like varying compressor displacement or compressor speed to adapt to the load. Variable frequency inverters are used to efficiently drive electric compressors.

The variable displacement trick is used in ones mechanically linked to internal combustion engines. It can vary the compression stroke to account for different load as well as different engine speed.

[geerlingguy]

Watching power draw on my Leaf with LeafSpy, the AC seems to use between 500-1000W (maybe more sometimes, but that's just off the top of my head from a few times running it while driving).

At the low end maybe achievable with a full rooftop covered in solar panels, but probably not adequate at 1kW+.

[danans]

> Does the extra 3-6 miles factor in the need to now run the AC much more aggressively because the car will be hot from sitting in the sun all day?

Most cars are already sitting in the sun all day.

[aziaziazi]

Not here in European cities where they’re either within a multi story park or in the side of a half day shaded tiny street.

[sdenton4]

Yah, it's a great point that the whole scheme is predicated on very questionable land use policy...

[parsimo2010]

You are not getting the 3-6 mile per day boost if your car is parked in the shade.

[gambiting]

What kind of European cities are you talking about lol, no offence but I hate this generalisation of "European" anything as if Southern Spain has the same culture and architecture as Poland or Lithuania.

[lupusreal]

To my thinking, the best use of a solar panel on a car is running a low power AC unit all the time whenever the car is in the sun. Parking in the shade often isn't possible.

[kgermino]

I don’t think you’d have to run the AC any more aggressively with the solar panels than with a traditional steel roof?

If you’re suggesting it wouldn’t work in a garage, that’s obviously true (and another factor in whether car solar makes sense) but many (most?) people park their cars outside during the day anyway. I for one can’t remember the last time I parked under cover

[singpolyma3]

That's still 3-6 fewer miles worth of charging to do from more expensive sources. Even if it can't come close to covering your full use it's still covering something

[IAmBroom]

Not if it won't recover it's own costs.

[cma]

It may still lose on this, but you would also want to include the externality costs that the consumer doesn't themselves bear for whether it is worth it overall.

[dzhiurgis]

Cost is convenience.

Manufacturing cost is like $40 cells, $20 electronics and $200 in glass fusion, mounting, etc.

My car eats 200W just being online so it would be useless to charge from solar.

[roamerz]

There is going to be a parasitic drag loss to figure into it as well. I think the only way to accurately calculate that would be in a wind tunnel or maybe an amp meter with a before and after installation under identical conditions.

[jeffbee]

The Prius Prime solar roof is a $610 option available only on the top XSE trim level, so a hypothetical buyer is paying ~$7500 to access this effectively negligible amount of energy.

ETA: and the fact that this option is tied to the significantly less efficient 19" wheel package, instead of the standard 17" wheels, means that this will never, ever be a net benefit.

[mattmaroon]

Not if they were getting that trim level anyway.

[AdamJacobMuller]

I just started doing this with my car, mostly to add a camera/temp monitoring for when I leave my dog in the kennel in the car (she's well watched over, please don't fret over it).

I'm hooking it up via starlink specifically so it works in remote areas with no cell coverage too.

Monitoring and proxying everything via an RPI as well. Victron DC-DC inverter to keep the bluetti battery pack charged with bluetooth relay boards so we can turn loads (camera/starlink/others) on/off programmatically (it only turns the starlink on when there's no good/known wifi for example).

Fun project, combines software dev (which I'm fairly good at) with hardware work (which I'm less) and my dogs (which I'm a big fan of).

[theoreticalmal]

It’s interesting to see how many people in this thread have the same idea. I’m doing something similar and learning that hardware is just plain hard

[ben_w]

The maths says that the *mean* number of miles driven by a vehicle is surprisingly low, and that tiling the surface of a car can get to about 80% of that *mean* in places where the car is just left out on the street and not shaded parking.

But!

That's a practical consideration at the level of "should a government require EV makers to design the roof, bonnet, doors etc. to be tiled in PV in order to reduce, but not eliminate, the induced extra demand on the grid" and definitely not "should I personally bolt a small, fixed, PV panel and inverter into my EV as an aftermarket DIY job?"

The former gets wind-tunnel tests for efficiency, QA, designed around all the other safety concerns cars have e.g. crash safety.

The latter, doesn't.

[jollyllama]

Very nice. How long does that tend to stay alive for? And what kind of cold weather conditions do you have to contend with?

[andoando]

If only we didnt start off with having 3000+ lbs of metal to move a 100-200lb person as a design limitation

[Aurornis]

> You add a lot of complexity for marginal gains. Peak time you get maybe 500W which doesn't go very far.

The complexity should not be overlooked. The PV panels add a lot of things that can fail: An additional layer that must be adhered or fastened the roof. Transparent panel covers that can become damaged in ways that aren’t as easy to repair as a rock chip in paint. Extra wiring that runs into the vehicle. A charging regulator. Systems to monitor that it’s all working and give the appropriate diagnostic codes if it fails.

Having worked on a lot of older and newer cars when I was younger, I’ve come to appreciate a degree of simplicity in vehicles. Modern electronics and vehicle systems are more reliable, but when the number of motors, sensors, and functions in a car goes up by 10X with all of the new features, a lot of little things start to fail in annoying ways as cars age out.

With solar I imagine old car owners would just ignore the system when it stopped working, but you’re still hauling all of that extra weight around for the lifetime of the car. That extra weight subtracts from your efficiency.

[secabeen]

The simplicity of EVs is one of their big strengths! Compare all the cooling, transmission, lubrication and fuel systems of an ICE car to the simple Electric Motor of an EV. Vastly simpler. As an end user, I see it to, my EV has no scheduled maintenance, whereas the ICE wants me to take it to the dealer every 20k miles.

[MisterTea]

Mechanically simple, yes. Electronically simple, no.

[jcgl]

But most of that electronic complexity is just as present in ICEs as in EVs, no? It’s not like most of it is in the drivetrain. All those auxiliary systems.

[MisterTea]

> All those auxiliary systems.

That's the problem. I don't need or want any of that extra nonsense. You can't replace the radio in these contraptions. The only thing a car needs is: drive-line control, braking/ABS/ESP, steering, HVAC. Electronics wise dump the lame glued on tablet look and just give me a single up front dash display and a double DIN cutout in the dash. HVAC only needs knobs. I hope a Chinese auto maker comes out with an EV that dumps all of this stupidity and just gives us cars again.

[secabeen]

Backup cameras (and the associated display in the dash) are a significant safety improvement. I wouldn't want to buy a car without one at this point, despite the increase in tech that it necessitates.

[jcgl]

Right, I agree. But the context was about the differences between ICE BEV. And they, afaik, have comparable bells and whistles.

[zdp7]

Take a look at https://www.slate.auto

[WorldMaker]

To be fair, modern PV cells are purely solid state (no moving parts, no lubricants or coolants), so a solar system shouldn't add a scheduled maintenance burden, just add to potential unscheduled maintenance costs in worst case scenarios.

[actionfromafar]

There was some car which used a small solar panel to pass fresh, cooler air into the cabin during sunny days. This both made the car more pleasant to enter and lowered the initial AC surge. I don't know if it also trickle charged the starter battery so it never could get completely depleted from just standing for longer periods. Both these things seemed worthwile.

[pyk]

The 2010 Prius IV had this as an option - one of my favorite cars due to low maintenance (the lowest maintenance visits per year for its era). The solar panel air vent circulation is a nice feature (even if slightly gimmicky) and I suspect extends the hybrid battery life as well by preventing some marginal battery heat death while parked.

The newest (2023+) Prius brought back the solar roof as an option - and this time it charges the battery (albeit marginally / but not bad for those that drive minimally).

[eldaisfish]

A more practical solution is to leave the windows slightly open so the hot air escapes.

[gabrielhidasy]

There are places where a marginally open window will invite vandalism, or rain, or bugs, or smoke and bad smells.

[eldaisfish]

ok, so in those places, you don't leave a window gap. in all the other places, you do.

[SoftTalker]

Not practical if rain is forecast.

[imp0cat]

The car already has a rain sensor so it can close the windows automatically. I do believe some VWs already do/did that.

[SoftTalker]

My car has that to activate the wipers. To say it works reliably is to lie.

[jvanderbot]

This is a perfect nerd snipe. I can't imagine any car owning (esp ev owning) engineer hasn't or wouldn't eventually think about "why can't I charge my car from my car".

[seltzered_]

You might like the series by youtuber 'Power of Light' where he packs solar panels in his car to charge his car to do a solar cannonball run from New York to California on those solar panels alone: https://m.youtube.com/playlist?list=PL9nfj0jfPXYBF8FO7sckzvV...

Can't remember how long it took, think a couple weeks at least?

[throwawaymaths]

he planned for 30 days and it wound up taking 60. he didn't strategize to minmax it, though? there was a lot of up and down and he stopped mostly at places where there were fun public camping parks. also he went solar -> battery -> ac -> tesla with an alternator to a standard tesla house plug iirc? whoo.

i think there was an interview later where he said, "yeah never doing that again" or something to that effect.

[tempestn]

Agreed. Using solar to power vehicles is great, but there's little benefit in the panels being on the vehicle. Put panels on your house, charge your EV, and you've got a solar powered vehicle (and house).

[HPsquared]

Say the car gets 4 miles per kWh. So a 500 W charging rate (neglecting losses) can be expressed as 2 mph.

Compare to a fast charger which will be several hundred mph.

[VBprogrammer]

Not sure if I've slipped a 0 here but 500w taken over the year, at say a 10% capacity factor, is still over 3500 miles of range per year. A fair bit short of the average mileage (in the UK somewhere around 10k) but still more significant than I expected. Of course 500w is a lot of solar for a car and 4 miles / kWh is also quite efficient.

[TrainedMonkey]

I think this is a flawed comparison. You only care about speed when driving, but charging we care about whenever the car gets sunlight. I would argue for most people car in sunlight time is a multiple of car driving time. Still pretty abysmal, but less bad than 2 mph.

[rfrey]

This is a good way to look at it, but perhaps a new unit, like range per hour? Since mph is alreday a unit of velocity.

[jermaustin1]

Range isn't a unit though, so it isn't actually telling you anything technical. Since range is a distance unit, it would still be "miles per hour" or "kilometers per hour" or "meters per second" or anything to let you know how long it will take to top up to full range.

Could be "%/minute" maybe, but that is less useful if you know you need to go 45 miles, you would want to know how many hours (or fraction there of) that would take.

[jameshart]

It expresses how many miles you can get in a given number of hours. It is a velocity.

[HPsquared]

Same dimensions, same units. Sure it can be expressed more specifically e.g. "miles of nominal range per hour". But it's still miles per hour to facilitate mental calculation.

[kgermino]

Miles (of range) per hour (of charge) is somewhat widely (and accurately) used as a metric for charging speed

[msgodel]

People don't seem to talk about Watt hours per mile much but when you're generating the power yourself it really matters. Tesla's model 3 is AFAIK one of the more efficient EVs and gets ~260 Watt hours per mile. With solar a good rule of thumb is to take the nominal rating for the panels you can point south and multiply it by 4 to get the approximate daily energy you'll generate in watt hours. If you could optimally park a car and let's assume you could cover it in a couple 100 Watt panels that would give you about four extra miles of daily range.

Maybe it's interesting if you live in a city and drive once a week.

[agumonkey]

I wonder if it would be OK-ish to build a very lightweight, very long, low powered solar "bus" (or a tram like chain). Just enough to roam around a city at 15-20mph for free.

You'd get enough surface to get ~4kW

[jerf]

There have been solar car competitions that colleges have been doing for decades. Here's a YouTube compilation of one that ran last week: https://www.youtube.com/watch?v=ZBin-oXBJzM

I think it can help calibrate people's intuitions about what you can expect out a pure-solar car.

You also need to remember that inside those shells is basically nothing but a driver. No AC, no seats for people beyond the bare minimum. And that's broad daylight. So you need to look at them doing 20-30mph and bear in mind that it's still not comparable to a street-legal sedan of a similar size doing 20-30mph... those cars are essentially as close to "a mobile cardboard box" as the competitors can make them.

You might be able to build something that people would agree is "a bus" that moves with a couple of people on board, but it probably will stop moving once it enters shadow. Anything that we'd call "a bus" is going to need a lot more physical material per unit solar input than those cars have. I'm not sure that even "moves with a couple of people on board" will necessarily end up being faster than those couple of people walking, either. It's effectively impossible to power a vehicle with its own solar footprint in real time. It also ends up difficult to use them to power batteries because having to move the additional mass of the batteries eats up the advantages of being able to gather power for larger periods of time. It's possible, because of course you can hook a car up to solar panels and eventually charge it, but you don't get very many miles-per-day out of it for what fits on the car itself alone if you work the math.

[mikepurvis]

Yup, was just going to link something like that— here's the University of Waterloo's solar car team's vehicles: https://www.uwmidsun.com/our-cars

And even those IIRC don't drive continuously. They drive for part of the day, then park them angled into the sun for the other part of the day to top up the batteries.

It's pretty hard to beat fixed panels + fast charging + parking your vehicle in a garage where it doesn't see the sun anyway (or get super hot).

[willvarfar]

Thx that was a really awesome video!

Commenting here to encourage other HNers to go watch it. Right now it has under 400 views and no comments.

[CerebralCerb]

It's an interesting idea. I did some napkin math based on the Solaris Urbino 18 bus. The buses have about 45 square meters of ceiling area (18m by 2.5m). Assuming efficient solar panels you could get 250w/sqm. That works out to 11.25 kwh/hour. The bus advertises with 600km of range with 800kwh of batteries so that is 1.33 kwh/km. Hence it could do ~8km/h on average when it is sunny.

The math does not really work out to a viable product with this bus, but it is not too far off. A city bus that has been purpose-built for low speed in urban areas without other traffic may work as it can make some sacrifices. For instance, since it runs much slower on average it would need smaller engines. It could also use more light-weight material since it won't need to handle high speed collisions. If it is just used for short distances within a city center it could also do away with seats. Lower speed should also lead to lower consumption.

The Solaris Urbino 18 weighs 17.5 tons curb weight. Assuming fuel consumption is pretty linearly related with weight and you could get it down to less than half, you could get a bus with a range of 10 miles per hour of charging. If it drove for 6 hours a day, but got charged for 12, 20 miles on average per hour is possible.

[agumonkey]

Yeah I wasn't clear enough but I was really thinking about the most limited form of "transportation", low speed, low weight, so minimal frame and no protections really. Basically a string of bus stops on wheels. Maybe an average speed of 13mph would be enough. That's 3 three times the average walking speed.

[AstralStorm]

Why bother? Put the charge station in the bus stop instead. They have a longer runtime to charge and the bus does not have to be slow. Potentially easier to maintain too.

[BobaFloutist]

Or even do light rail and electrify the tracks with a solar network wherever you want.

[sdeframond]

Would that be more interesting with tram because of the low-friction wheels?

I imagine that could be viable in, say, Dubai or some other extremely sunny place ?

[lm28469]

Why bother ? Have the solar panels on top of the tram warehouse, use the tram batteries for storage, swap empty ones for full ones when needed. If the solar array is down use the grid. That way you divid points of failure instead of multiplying them

[immibis]

Or... power the tram lines from the grid and feed solar power into the grid somewhere else.

Trams use fixed infrastructure, including overhead power lines. I'm sure they must exist somewhere, but battery-powered trams are not popular.

[cesarb]

> I'm sure they must exist somewhere, but battery-powered trams are not popular.

Yes, they do exist. The Alstom Citadis at Rio de Janeiro, which I take often, uses a supercapacitor for small pieces of its route (mostly crossings where the third rail would be damaged too often by vehicle traffic, or be impractical); according to the Wikipedia article (https://en.wikipedia.org/wiki/Alstom_Citadis), the Alstom Citadis at Nice uses batteries for parts of its route (https://www.railway-technology.com/projects/nice-trams/). I'm sure there are others.

[bestouff]

4kW on a bright sunny day, for a few hours around noon. Even my small EV outputs 100kW when floored, and 4kW doesn't get it very fast.

[lazide]

In direct, unshaded sunlight. Which is the opposite of any significant sized city I’m aware of.

[PunchyHamster]

We have trams. We don't need to make worse trams

[Zigurd]

I suspect the lightweight, and hence low power requirements, are the correct part of the hypothesis. But making the vehicle as big as a bus implicates a lot of weight. Maybe a solar charging cargo bike fairing would have some benefit, but that's an expensive bike and it will tend to get stored indoors.

[BizarroLand]

Maybe an electric assisted pedal bus with a solar roof would make sense.

Very location specific, might do wonders in Cancun or San Francisco or Vegas, not so much in Gatlinburg or Seattle or anywhere where there is not a lot of tourism or where there is a lot of rain or that has a long snowy season.

[quickthrowman]

One horsepower is roughly equivalent 754 watts. 4000/754 = 5.305 horsepower. Even tripling it only gets you to 15 HP.

I am not an automotive engineer but I doubt that is enough power for a bus that people can ride.

[johannes1234321]

Well, if you have a fixed route you are not limited by space on the vehicle to put solar on, but can provide electricity via a rail or wire or something and then gather energy on some larger Solarstation or from wind turbines or what else comes to mind.

Then you can reduce rolling resistance by using steel tracks and steel wheels ...

... and oh, you have invented the tram/light rail ;)

(But even with solar you need to finance the construction and maintenance, even the slow vehicle need some ... thus either tax finance or charge fares or mix income)

[PunchyHamster]

Only advantage is if you use car rarely, park outdoors and don't want onboard battery to drain, tho way smaller panel needed to cover that

[marcosdumay]

Yep. A solar car ceiling seems great to make EVs more reliable on the hands of people that only charge them rarely or may travel to the middle of nowhere and can get surprised by battery faults.

Those are a very small share of car owners, and EVs are nowhere close to the market penetration to care abut them. But it will eventually make sense.

[walrus01]

I agree on this. Using the pvwatts calculator for a very rough estimate of cumulative kWh produced per *month*, a theoretical 380W panel on top of a car that is in perfect sunshine from sunrise to sunset, never shaded or obstructed, on a car in the sunny climate of San Diego CA will produce the following:

61 kWh per month in the best month of the year (August)

39 kWh per month in the worst month of the year (December)

As you can see from this, the kWh per day is quite minuscule, not enough to charge a car to go any appreciable distance.

[chiph]

I believe that solar panels were an option on the Maybach 62S, and they would run the ventilation fan while you were parked so you wouldn't return to a hot car after going to the store.

Like everyone else has said - there just isn't enough area on the top surfaces of a car to do any noticeable charging.

[walrus01]

If you were to theoretically have a perfect 400W PV panel on top of a car, and left in direct sunlight, it might be enough to run a medium sized peltier/TEC cooling unit to somewhat cool down the car while you leave it parked. Or a very small heat pump. Would definitely add a lot of extra cost in manufacturing and complexity.

[gabrielhidasy]

Or just keep the car fan running and use the existing AC system (in ventilation mode, no compressor) to keep the car just as hot as outside (instead of much hotter). If you have some spare power maybe even run the AC when the key gets back in range.

[gus_massa]

Using "270Wh/mile" from another comment,

(61kWh/month) / (270Wh/mile) / (31day/month) = 7.3mile/day =~ 11.7km/day

(39kWh/month) / (270Wh/mile) / (31day/month) = 4.7mile/day =~ 7.5km/day

My conmute is like 3 or 7 miles (4 or 11 km), depending on where I have to go.

Anyway, I expect that a rooftop installation is much more efficient.

[walrus01]

The rough estimate calculation for the theoretical 39 to 61 kWh per month are for a perfectly mounted, south facing, 15 degree tilted PV panel such as might be on the roof of a warehouse, or in a field somewhere. With no buildings or trees or shade obstructions around it. And perfectly exposed to sunlight from the moment of sunrise all the way to sunset. That's the 'default' assumptions built into pvwatts for calculating a fixed installation PV site.

On an actual car that parks under trees, in parking garages, beside buildings in the shade, etc, the actual production would be much less. Not to mention the panel would be 'flat' on the roof and rarely if ever angled facing south, unless you happened to park on a hill with the roof of the car angled south...

It's also not possible to say that a theoretical 39kWh can be turned into so many miles at 270Wh/mile because it's not a perfectly efficient system, I'd guess at least 15-20% would be lost to heat in charging the battery and DC-DC conversion.

[bestouff]

60kWh may be enough for occasional short trips.

[jillesvangurp]

The math is biased towards when you are using the vehicle. The solar panels also work when you aren't using the vehicle. They work from when the sun comes up until it goes down. And actually most people don't actually use their cars most of the time. It's just sitting there parked doing nothing well over 90 percent of the time. And especially hybrids have tiny batteries to begin with. Instead of charging those burning petrol, you could be partially charging those with solar.

If you get 400W watt performance for a few hours per day, that's maybe a couple of kwh per day. 2 would be alright. 4 would be amazing. 6 probably not that likely unless you live in a very sunny place. Most decent EVs do at least 3 miles per kwh. So, you get maybe 6-12 "free" miles per day. Maybe more with an efficient one. Up to 20 miles even.

Most commute round trips aren't that long. You are might need more power than that. But not a lot. You could be cutting how often you charge by some meaningful percentage. It's not going to be that useful on a long journey. But most people don't do those all the time but they drive small distances on a daily basis. Imagine you drive to work, and back maybe covering 20 miles. You go to sleep, and the car is back at 100% charge. Because you only used a few kwh driving there and back and the car had plenty of time parked to collect those back because the weather is nice. Or maybe it got to 95%. The difference is meaningless because you only use a few percent on a given day. Basically you'd be charging a bit less often and stretch existing charges a bit longer.

If you have a 60kwh battery and you get 2kwh per day from the sun, that's 1 full charge per month. Most people would charge maybe 2-4 times per month. So that's a meaningful amount. Cutting them amount of power that you have to pay for by 25 or more percent can be interesting. I think for most the savings aren't going to be dramatic. But it's nice that the car just sits there slowly topping its battery up without you having to worry about it. That's convenient.

[bongodongobob]

When I'm not using my vehicle, it's in my garage.

[sevensor]

Can you comment more on the complexity? Like, is it running wire harnesses everywhere, is it the power electronics, cooling, mechanical mounting, something else, all of the above?

[janosch_123]

Of course. It is an intriguing idea, but a local maximum.

- The panel sits at open-circuit voltage of 48V

- That then needs to be converted/boosted to 400V (conversion loss)

- The converter needs to talk to the BMS to make sure batteries can be charged at this moment (component that is live all the time and is a current draw)

- Need to think about it, but you want another set of contactors between panel and HV-Bus where the battery sits (current draw)

1km of driving is 150Wh so 1kWh gets you 6.6km or 4.1 mi

Let's be generous and say you have a 500W panel(punchy) for 8 hours at full blast (doesn't happen), you get 500W x 8 hrs = 4kWh. Lets say isolated converter loses you 10% so you are at 3.6kWh Thats 24km or 15mi of driving in perfect conditions.

2x Gigavac contactors, keep them closed costs you 24W, so that lowers the input further to 476W * 8hrs = 3.8kWh, less 10% = 3.42kWh ...

Someone who studied EE might be able to make this more accurate. Back of the napkin math, not totally impossible, but not worth adding it for a trickle charge. Adding components that can break, adding weight etc.

There are interesting solar cars out there where you reduce the weight heavily and fold out big solar sails. Then you are getting somewhere, for a city car you don't have enough surface. For an SUV or American Style Flatbed truck you have so much weight it's not worth it either.

[Aachen]

> not totally impossible, but not worth adding it

I don't drive 24km per day, and don't have a good way to get to the train station other than by car. The bus is too tight, they miss each other often. Cycling isn't safe between towns, you have to basically go on a highway without any separation (yes that's legal in Germany to cycle on, as there is no other way than perhaps a farmer's grass path to go between towns, so they don't call them highways but cars drive highway speeds - or more, if they don't stick to the limit). I also don't have charging infrastructure or a driveway. A vehicle that does those couple km a few times per week without needing to drive elsewhere to charge gets me a long way. Charge me up, Scotty

I've looked into this and the moment the Aptera ships (probably never but here's for hoping) I'm buying my first car. I've looked critically at the range they assume you get at my latitude and it would keep topped up for enough months of the year that it's totally worth it (maybe it was even year-round because they're so efficient, I don't remember now, but I'm also okay charging it thrice a year)

[labcomputer]

But 24 km per day is under ideal conditions (perfectly sunny day, mid-latitude, panels angled southward) and 500W requires 2 square meters of panels[1].

Unless you own a big American pickup truck, it's hard to see where those panels fit on the car. And if you do own a big American pickup truck, you will not achieve the 150 Wh/km assumed by the GP (it will be more like double that). GP also used quite optimistic loss figures for conversion.

It begs the question: Why not a Nissan Leaf and solar panels on your (home) roof?

[1] Only 1000 W of solar energy falls on each square meter of the earth's surface at noon. The best commercially available solar panels have about 25% efficiency converting light to low voltage DC. This means you need a flat surface of about 2 square meters directly facing the sun to collect 2000 W of light, which will achieve 500 W of electrical power.

[Aachen]

Why not on your home roof: because I don't have a home roof or a place to run the cable to the car. I could ask the landlord if they'll allow me to pay for solar panels on their building but I still don't have a driveway as mentioned

Why doesn't the Leaf have it: afaik the leaf is a normal car, not like the Aptera that I'd be looking to buy. Even the Lightyear One was claiming to be more efficient than normal but pretty disappointing in how much range you get from the solar roof. Still more than zero though, so yeah ask them why they don't sell that variant ¯\_:|_/¯ While we're at it, let's find out why there isn't a smartphone that fits my niche (modern chipset with dual frequency GNSS for less than twice the price that competitors charge for it, operable with one average-sized hand, sdcard slot, and ideally a headphone jack but at this point I'd settle just for a bit of storage and really don't feel like it should be a hard-to-find device) or a laptop that makes sense (arrow keys and no annoying tapered edges that don't fit a network port, for example, already limits your options to a tiny percentage of the available systems)

[sevensor]

Thanks! That helps clarify the engineering challenges. There’s never a free lunch!

[bee_rider]

Maybe an RV could be covered with solar? The top is much bigger, and if it isn’t charging fast enough you can always pull over and have lunch while the battery catches up.

[mikepurvis]

But an RV is also way bigger and heavier.

RV panels make sense for the boondocking use case, where you want to charge computers or power a satellite internet terminal or something, but I can't imagine actually trying to drive on that trickle of juice.

[kgermino]

Agreed. There’s an EV camper van with rooftop solar. IIRC it gets about 1000W peak, which isn’t bad for the home batteries but is basically nothing for the high voltage drive system

[amluto]

How about charging your house batteries, which power fans and lights and perhaps cooking and A/C? This kind of solar setup can be rather cheap and quite effective.

[mikepurvis]

Yes, that's what I'm meaning. AIUI off-grid camping tends to be more limited by the drinking water supply more than electricity, but if collecting solar power lets you avoid running the generator quite as often, that sounds like a win.

[wing-_-nuts]

This and you could also charge an ebike

[thijson]

Boats have been doing this for ages.

https://www.youtube.com/watch?v=BmbH-TAM2Wk&list=PLQp8FoQ4t-...

[masklinn]

It’s always going to be anecdotal. I reckon a mid size RV (say upper class B) will have 1500-2000W of solar capacity, if it’s really boxy. It’s going to have the aerodynamics of a brick. Meaning you’ll be lucky to get 1mi/kwh at highway speed, maybe 2~2.5 if you keep under 30.

So you’ll be charging at 2~5mi/h, if the sun is shining straight overhead.

It’ll count for something if you park the RV in the sun for a week as you camp somewhere, but on the road it gives you some limping ability and that’s about it. The main benefit is not running the AC off of the engine.

[driverdan]

It doesn't make sense to power any vehicle with onboard solar. There are no electric RVs yet because the batteries required to have any amount of range are cost prohibitive and heavy.

I put 1800W on my RV and that's covering the roof end to end. I'd guess it'd be enough for something like 1-2 miles a day on an electric drive train, assuming you don't use power for anything else.

[masklinn]

> I'd guess it'd be enough for something like 1-2 miles a day on an electric drive train

It's probably more 10~20, possibly as much as 30, if it's a long and sunny summer day FWIW.

For references:

- the F150 lightning gets close to 2 miles / kWh on average, ~1.5 at highway speeds but as much as 3~4 at consistent low speeds

- on the other side of an RV, Volvo markets their FH Electric (cabover semi) for 1.1kWh/km — 0.7 mi/kWh — at 80km/h (50mph), DAF/Innovate UK's Battery Electric Truck Trial yielded similar results (1.08kWh/km over 287000 km), it's also close to the numbers of the electric trucker in their very recent MAN eGTX video (0.83 kWh/km = 0.75 mi/kWh)

[driverdan]

It averages something like 12mpg diesel. This is a 25' long, 16k lb vehicle. There's no way it would get 2mi/kWh. 0.7 mi/kWh sounds closer to what it'd be.

But that's assuming you use no power. When I was living in it I was using 50% to 100% of the solar output a day.

[SideburnsOfDoom]

Even better to get a fixed structure such as a garage or carport, that keeps the vehicle safe and out of the sun, and cover that in Solar.

It has larger surface area, doesn't weight the vehicle down at all even if it's built in a less weight-efficient way, and the vehicle doesn't need to be exposed to the elements.

[ErikHuisman]

Who lunches for several days/weeks? logically you would charge high speed through a plug with energy generated by panels that are much more efficiëntly (money+yield) placed and not have to carry around.

[sixothree]

People are absolutely starting to populate their RVs with solar. What I've seen so far is just a few panels - around 600 watts. Usually connected to a battery separated from the RV wiring.

[chris_va]

One can now get (flexible-ish) multi-junction PV (say 29% efficiency) from the factory for under $1/W. Still a higher price than the $0.2/W, lower efficiency panels, but when I messed with panels I felt like we were living in the future.

Anyway, one could also set up the panel to output a much higher voltage by having the factory wire cells in series (though how well that trades off with partial shading for a car roof I have no idea, and I have no idea the minimum quantity required to get that).

... but I agree, even with all that, it seems like a stretch to make it work.