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Why Teslas don’t—and can’t—have solar roofs [1] The top of an electric car has maybe 3–5 square meters of flat space. Solar panels, even at high noon, usually

Why Teslas don’t—and can’t—have solar roofs [1] The top of an electric car has maybe 3–5 square meters of flat space. Solar panels, even at high noon, usually only produce about 200 watt-hours per square meter. The most efficient production electric vehicles today (probably the Hyundai Ioniq and the Tesla Mod 3) would only be able to travel 2–4 miles on that amount of electricity…in an hour. Most people could walk faster. Financially, the cost of the panels and electronics, R&D and assembly would never pay for itself in the life of the vehicle, compared to charging from the wall in your garage. While it might be nice to have for cars that sit there for a while or in the apocalypse. Right now it is better to use home solar panels or charging, as they have more surface area. Or maybe build parking structures with panels that can charge with more area. Maybe one day when they are small enough or can pull in sun into a smaller surface areas it will make sense. Solar panel cars would probably also really up insurance rates and add more safety work/R&D. [1] https://qz.com/1482588/why-teslas-dont-and-cant-have-solar-roofs/ https://qz.com/1482588/why-teslas-dont-and-cant-have-solar-r...

hrktb 7y ago
Isn't it a case of the better killing the good ? From your excerpt is seems electric panels still give 2-4 miles of range per hour of charge. Even taking into account the added weight, it's a net positive so why dismiss the idea completely ?
- nullymcnull 7y ago
  Read it again: > would only be able to travel 2–4 miles on that amount of electricity… in an hour. Most people could walk faster.
  - hrktb 7y ago
    I got it wrong the first time, then got it at the second take, thought I had it wrong again reading your comment and turned around again. The writing was weirdly misleading. > Solar panels, even at high noon, usually only produce about 200 watt-hours per square meter. > The most efficient production electric vehicles today (probably the Hyundai Ioniq and the Tesla Mod 3) would only be able to travel 2–4 miles on that amount of electricity…in an hour. Most people could walk faster. From what I get: - in the best conditions the panels could produce 200W in an hour - 200W would allow for 2-4miles on the most efficient cars The author's point seems to be that if the solar panel was the only source of energy the car would be slower that walking. It's different from seeing it as an additional source of electricity, just like regenerative braking is for instance.
    - labmachine 7y ago
      > - 200W would allow for 2-4miles on the most efficient cars Sorry, but that's wrong. They meant 2-4 miles per 1 kWh, not per 200 Wh. Very good EVs get about 4-4.5 miles / kWh at optimal speed (usually between 25 and 35 mi/hr). So, best case, 200 Wh is good for about 1 mile, assuming you can deliver the 200 Wh over a period of 2 minutes. However, 200 W can't push the car at 1 mi/hr because of "vampire" losses: Power for accessories and vehicle systems (like power-assisted steering, power-assisted friction brakes, the ABS controller, the airbag monitor, etc). 200 W of power isn't just slower than walking: In a normal-sized car, you will literally go 0 mi/hr. More importantly, 200 W/m^2 is the best-case scenario at noon, in the tropics, on a cloudless day, with no shadows, using expensive, high-efficiency panels aimed squarely at the sun. As soon as any of those qualifiers is not met, power drops precipitously. In particular, the atmosphere absorbs a large amount of light: In space, solar insolation is about 1300 W/m^2. At earth's surface in the tropics, solar insolation is about 1000 W/m^2 at noon. At higher latitudes, or other times of the day, solar insolation is lower. A good rule of thumb is that (for Europe and North America) the total insolation over a full day is about 5000 Wh/m^2. So, a solar panel on a sun-following mount is limited to about 1000 Wh/m^2/day, again assuming no clouds. For a fixed solar panel, such as on the hood of a car, you'll get about half of that: 500 Wh/m^2/day. Remember, you need 1000 Wh to go 4-5 miles.
      - bloak 7y ago
        "Remember, you need 1000 Wh to go 4-5 miles." That "4-5" implies a lot of accuracy, but I would have thought that the distance you can travel on 1000 Wh would vary by orders of magnitude depending on the speed, terrain and road surface. However, if you are moving a 1 tonne vehicle up a 5% gradient then you won't do more than 4.5 miles on 1000 Wh, if I've calculated that correctly.
- majewsky 7y ago
  > Even taking into account the added weight, it's a net positive You just gloss over the most important part of the question right there. At a charging rate that low, even a small amount of added weight may completely destroy the plausibility of this idea.
  - kuprel 7y ago
    If your daily commute is short, solar panels could provide enough energy to not worry about charging
    - usrusr 7y ago
      Or if you don't have a daily commute at all. Perfect for those who could switch to the occasional taxi or rental but stick to owning a car due to cultural inertia, perfect as that fallback family car that only gets used when the family fleet is fully saturated, perfect for the plugin hybrid that never gets plugged in due to the inconvenience of keeping two kinds of energy storage stocked.
  - hrktb 7y ago
    If it can generate enough energy to move the 1 ton vehicle for 2~4 miles, it seems it's pulling its weight, literally.
- m4rtink 7y ago
  That actually sounds pretty useful - say the car gets 10 hours of perfect iradiation on a good day, that's 20-40 miles, that could be a sizeable part of a daily commute, not to mention cases where you have the car standing still for a few days.
  - 0-_-0 7y ago
    That would be more like 4 hours. A car solar panel would be horizontal, and it wouldn't track the sun when it's lower on the horizon. The 200W figure is only for noon.
  - adrianN 7y ago
    Solar panels in Germany produce about 10% of peak power if you average over the year. Those panels are generally installed in the correct orientation to maximize efficiency. Panels on a car will likely perform much worse.
- mtrycz2 7y ago
  Honestly it seems like a nice idea. The idea that a car can go _entirely_ on solar on it's roof is clearly off-limits for now, but that doesn't disqualify the idea entirely. Say I use my car for the daily commute and I'm some 10 miles away from office. I work 8 hours in the office in daytime, and leave my car in the parking. For when I'm going back home, my battery would be topped up. I'd need to recharge my car much less, once a month maybe. Also, say you can do 300 miles with a full charge. Depending on how fast you go, you'll do those 300 miles in 5 to 10 hours. So on average, a the solar aid would give you some additional 15 to 30 miles, ie 5%-10%. Which sounds nice to me. All with the current technology. Obviously there are tradeoffs, in R&D, price and weight. But it seems like worth pursing, log-term.
  - rabscuttler 7y ago
    The folk building Lightyear One [0], a fully solar powered prototype car, would disagree with the assertion that it is off limits for now. Obviously the car is quite different from a Prius or any other modern car. [0] https://en.m.wikipedia.org/wiki/Lightyear_One https://en.m.wikipedia.org/wiki/Lightyear_One
    - jerf 7y ago
      I would be very surprised if that car could meet crash-test standards to be sold at large scale. If you don't have to meet crash test standards, you can make a much lighter car than you can today.
Tade0 7y ago
I have issue with this: these panels would still capture enough energy to drive at least 2-3k kilometers annually - that's at least 10% of your typical annual mileage, so nothing to sneer at.
- jdnenej 7y ago
  It doesn't get around the fact that it still makes way more sense to put those panels on your house roof and charge from that. The house roof is in direct sunlight for much longer than a car.
  - kalleboo 7y ago
    That is an excellent point - if you have a house. It might make more sense for people who have to live with streetside parking?
  - Liquid_Fire 7y ago
    Why not do both?
    - jdnenej 7y ago
      Because very few people have covered every last spot on their roof so you would be designing a product suitable for very few people
      - kungtotte 7y ago
        Very few people have electric cars to begin with, and they're really expensive compared to ICE vehicles so you're already talking about a demographic that's demonstrated a willingness to pay a premium for driving an electric car. If Tesla offered a "Solar Panel Kit" that made their cars even a few percent more efficient it would sell like hot cakes.
      - Tade0 7y ago
        Or for people who don't own a roof at all - plenty of such people in cities.
raducu 7y ago
I don't drive to work, but 2 miles per hour would be crazy good for me, as I live inside a city and my total commute is exactly 18 miles, which would mean my commute would be "free" (not taking into account the cost of the panels or the vehicle).
- adrianN 7y ago
  It's also unrealistically good as you don't get high noon insolation for 9 hours a day.
  - Retric 7y ago
    You do however get some power for more than 9 hours per day. Also, panels can go well past 200w/m at a theoretically possible 600w/m the numbers look rather different. Cars are only going 12-15k miles a year so hit 40miles a day and you generally stop needing charging stations.
    - jartelt 7y ago
      The amount of power incident from the sun is roughly 1000 W/m^2 in a sunny area. Good Silicon PV panels will capture 20% of that power generating 200 W/m^2 at peak. Getting anything more than that is going to be too expensive for a car application. Pulling 600 W/m^2 from a PV panel may be theoretically possible, but the world record efficiency for a PV device is still less than 50%. To get above 40% efficiency you need to use very expensive multijunction devices and solar concentrators (i.e. not going to work on a car).
      - Retric 7y ago
        I generally agree today, but that says little about the long term. As solar panels have gotten cheaper installation costs have become more important which has pushed up panel efficiency. Cheap but good panels are up to ~23% efficiency where they where ~18% fairly recently. What’s interesting is these trends seem to be continuing with serious research into various affordable multi junction cells going on.
projektfu 7y ago
I drive a gen1 Chevrolet Volt. Plugged into a 110V outlet it adds about 4 miles range in an hour. This is enough to make its benefits worthwhile.

hrktb 7y ago

Isn't it a case of the better killing the good ? From your excerpt is seems electric panels still give 2-4 miles of range per hour of charge. Even taking into account the added weight, it's a net positive so why dismiss the idea completely ?

nullymcnull 7y ago
Read it again: > would only be able to travel 2–4 miles on that amount of electricity… in an hour. Most people could walk faster.
- hrktb 7y ago
  I got it wrong the first time, then got it at the second take, thought I had it wrong again reading your comment and turned around again. The writing was weirdly misleading. > Solar panels, even at high noon, usually only produce about 200 watt-hours per square meter. > The most efficient production electric vehicles today (probably the Hyundai Ioniq and the Tesla Mod 3) would only be able to travel 2–4 miles on that amount of electricity…in an hour. Most people could walk faster. From what I get: - in the best conditions the panels could produce 200W in an hour - 200W would allow for 2-4miles on the most efficient cars The author's point seems to be that if the solar panel was the only source of energy the car would be slower that walking. It's different from seeing it as an additional source of electricity, just like regenerative braking is for instance.
  - labmachine 7y ago
    > - 200W would allow for 2-4miles on the most efficient cars Sorry, but that's wrong. They meant 2-4 miles per 1 kWh, not per 200 Wh. Very good EVs get about 4-4.5 miles / kWh at optimal speed (usually between 25 and 35 mi/hr). So, best case, 200 Wh is good for about 1 mile, assuming you can deliver the 200 Wh over a period of 2 minutes. However, 200 W can't push the car at 1 mi/hr because of "vampire" losses: Power for accessories and vehicle systems (like power-assisted steering, power-assisted friction brakes, the ABS controller, the airbag monitor, etc). 200 W of power isn't just slower than walking: In a normal-sized car, you will literally go 0 mi/hr. More importantly, 200 W/m^2 is the best-case scenario at noon, in the tropics, on a cloudless day, with no shadows, using expensive, high-efficiency panels aimed squarely at the sun. As soon as any of those qualifiers is not met, power drops precipitously. In particular, the atmosphere absorbs a large amount of light: In space, solar insolation is about 1300 W/m^2. At earth's surface in the tropics, solar insolation is about 1000 W/m^2 at noon. At higher latitudes, or other times of the day, solar insolation is lower. A good rule of thumb is that (for Europe and North America) the total insolation over a full day is about 5000 Wh/m^2. So, a solar panel on a sun-following mount is limited to about 1000 Wh/m^2/day, again assuming no clouds. For a fixed solar panel, such as on the hood of a car, you'll get about half of that: 500 Wh/m^2/day. Remember, you need 1000 Wh to go 4-5 miles.
    - bloak 7y ago
      "Remember, you need 1000 Wh to go 4-5 miles." That "4-5" implies a lot of accuracy, but I would have thought that the distance you can travel on 1000 Wh would vary by orders of magnitude depending on the speed, terrain and road surface. However, if you are moving a 1 tonne vehicle up a 5% gradient then you won't do more than 4.5 miles on 1000 Wh, if I've calculated that correctly.
majewsky 7y ago
> Even taking into account the added weight, it's a net positive You just gloss over the most important part of the question right there. At a charging rate that low, even a small amount of added weight may completely destroy the plausibility of this idea.
- kuprel 7y ago
  If your daily commute is short, solar panels could provide enough energy to not worry about charging
  - usrusr 7y ago
    Or if you don't have a daily commute at all. Perfect for those who could switch to the occasional taxi or rental but stick to owning a car due to cultural inertia, perfect as that fallback family car that only gets used when the family fleet is fully saturated, perfect for the plugin hybrid that never gets plugged in due to the inconvenience of keeping two kinds of energy storage stocked.
- hrktb 7y ago
  If it can generate enough energy to move the 1 ton vehicle for 2~4 miles, it seems it's pulling its weight, literally.
m4rtink 7y ago
That actually sounds pretty useful - say the car gets 10 hours of perfect iradiation on a good day, that's 20-40 miles, that could be a sizeable part of a daily commute, not to mention cases where you have the car standing still for a few days.
- 0-_-0 7y ago
  That would be more like 4 hours. A car solar panel would be horizontal, and it wouldn't track the sun when it's lower on the horizon. The 200W figure is only for noon.
- adrianN 7y ago
  Solar panels in Germany produce about 10% of peak power if you average over the year. Those panels are generally installed in the correct orientation to maximize efficiency. Panels on a car will likely perform much worse.
mtrycz2 7y ago
Honestly it seems like a nice idea. The idea that a car can go _entirely_ on solar on it's roof is clearly off-limits for now, but that doesn't disqualify the idea entirely. Say I use my car for the daily commute and I'm some 10 miles away from office. I work 8 hours in the office in daytime, and leave my car in the parking. For when I'm going back home, my battery would be topped up. I'd need to recharge my car much less, once a month maybe. Also, say you can do 300 miles with a full charge. Depending on how fast you go, you'll do those 300 miles in 5 to 10 hours. So on average, a the solar aid would give you some additional 15 to 30 miles, ie 5%-10%. Which sounds nice to me. All with the current technology. Obviously there are tradeoffs, in R&D, price and weight. But it seems like worth pursing, log-term.
- rabscuttler 7y ago
  The folk building Lightyear One [0], a fully solar powered prototype car, would disagree with the assertion that it is off limits for now. Obviously the car is quite different from a Prius or any other modern car. [0] https://en.m.wikipedia.org/wiki/Lightyear_One https://en.m.wikipedia.org/wiki/Lightyear_One
  - jerf 7y ago
    I would be very surprised if that car could meet crash-test standards to be sold at large scale. If you don't have to meet crash test standards, you can make a much lighter car than you can today.

Tade0 7y ago

I have issue with this: these panels would still capture enough energy to drive at least 2-3k kilometers annually - that's at least 10% of your typical annual mileage, so nothing to sneer at.

jdnenej 7y ago
It doesn't get around the fact that it still makes way more sense to put those panels on your house roof and charge from that. The house roof is in direct sunlight for much longer than a car.
- kalleboo 7y ago
  That is an excellent point - if you have a house. It might make more sense for people who have to live with streetside parking?
- Liquid_Fire 7y ago
  Why not do both?
  - jdnenej 7y ago
    Because very few people have covered every last spot on their roof so you would be designing a product suitable for very few people
    - kungtotte 7y ago
      Very few people have electric cars to begin with, and they're really expensive compared to ICE vehicles so you're already talking about a demographic that's demonstrated a willingness to pay a premium for driving an electric car. If Tesla offered a "Solar Panel Kit" that made their cars even a few percent more efficient it would sell like hot cakes.
    - Tade0 7y ago
      Or for people who don't own a roof at all - plenty of such people in cities.

raducu 7y ago

I don't drive to work, but 2 miles per hour would be crazy good for me, as I live inside a city and my total commute is exactly 18 miles, which would mean my commute would be "free" (not taking into account the cost of the panels or the vehicle).

adrianN 7y ago
It's also unrealistically good as you don't get high noon insolation for 9 hours a day.
- Retric 7y ago
  You do however get some power for more than 9 hours per day. Also, panels can go well past 200w/m at a theoretically possible 600w/m the numbers look rather different. Cars are only going 12-15k miles a year so hit 40miles a day and you generally stop needing charging stations.
  - jartelt 7y ago
    The amount of power incident from the sun is roughly 1000 W/m^2 in a sunny area. Good Silicon PV panels will capture 20% of that power generating 200 W/m^2 at peak. Getting anything more than that is going to be too expensive for a car application. Pulling 600 W/m^2 from a PV panel may be theoretically possible, but the world record efficiency for a PV device is still less than 50%. To get above 40% efficiency you need to use very expensive multijunction devices and solar concentrators (i.e. not going to work on a car).
    - Retric 7y ago
      I generally agree today, but that says little about the long term. As solar panels have gotten cheaper installation costs have become more important which has pushed up panel efficiency. Cheap but good panels are up to ~23% efficiency where they where ~18% fairly recently. What’s interesting is these trends seem to be continuing with serious research into various affordable multi junction cells going on.

projektfu 7y ago

I drive a gen1 Chevrolet Volt. Plugged into a 110V outlet it adds about 4 miles range in an hour. This is enough to make its benefits worthwhile.