15 ms·

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 ch

• walrus01 1y ago
  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 1y ago
    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 1y ago
      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 1y ago
        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.
  • bestouff 1y ago
    60kWh may be enough for occasional short trips.
  • gus_massa 1y ago
    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 1y ago
      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.
• agumonkey 1y ago
  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
  • bestouff 1y ago
    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 1y ago
      In direct, unshaded sunlight. Which is the opposite of any significant sized city I’m aware of.
  • deleted 1y ago
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  • PunchyHamster 1y ago
    We have trams. We don't need to make worse trams
  • CerebralCerb 1y ago
    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.
• actionfromafar 1y ago
  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 1y ago
    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 1y ago
    A more practical solution is to leave the windows slightly open so the hot air escapes.
    • SoftTalker 1y ago
      Not practical if rain is forecast.
      • imp0cat 1y ago
        The car already has a rain sensor so it can close the windows automatically. I do believe some VWs already do/did that.
        • SoftTalker 1y ago
          My car has that to activate the wipers. To say it works reliably is to lie.
    • 1y ago
• pchew 1y ago
  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.
  • jollyllama 1y ago
    Very nice. How long does that tend to stay alive for? And what kind of cold weather conditions do you have to contend with?
  • barnas2 1y ago
    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.
    • jeffbee 1y ago
      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 1y ago
        Not if they were getting that trim level anyway.
    • 1y ago
• HPsquared 1y ago
  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.
  • rfrey 1y ago
    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.
    • deleted 1y ago
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    • HPsquared 1y ago
      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.
    • jameshart 1y ago
      It expresses how many miles you can get in a given number of hours. It is a velocity.
    • jermaustin1 1y ago
      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.
    • kgermino 1y ago
      Miles (of range) per hour (of charge) is somewhat widely (and accurately) used as a metric for charging speed
  • VBprogrammer 1y ago
• jvanderbot 1y ago
  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_ 1y ago
    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=PL9nfj0jfPXYBF8FO7sckzvVHnQsFVL6Yu https://m.youtube.com/playlist?list=PL9nfj0jfPXYBF8FO7sckzvV... Can't remember how long it took, think a couple weeks at least?
    • throwawaymaths 1y ago
      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.
• PunchyHamster 1y ago
  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 1y ago
    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.
• sevensor 1y ago
  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 1y ago
    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 1y ago
      > 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)
• msgodel 1y ago
  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.
• bee_rider 1y ago
  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.
  • ErikHuisman 1y ago
    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.
  • mikepurvis 1y ago
    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 1y ago
      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 1y ago
      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.
      • wing-_-nuts 1y ago
        This and you could also charge an ebike
      • mikepurvis 1y ago
        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.
• chris_va 1y ago
  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.
• Aurornis 1y ago
  > 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 1y ago
    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 1y ago
      Mechanically simple, yes. Electronically simple, no.
      • jcgl 1y ago
        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 1y ago
• jillesvangurp 1y ago
  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 1y ago
    When I'm not using my vehicle, it's in my garage.
• deleted 1y ago
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• tempestn 1y ago
  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).