[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.

[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).

[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.

[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.

[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?

[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.

[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.

[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.

[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.

[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.

[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.

[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.

[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.

[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.

[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.