# Is it feasible to put solar panels on the roofs of cars? **URL:** \ **Category:** Factual Questions\ **Created:** [September 22, 2018, 2:00pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731 "2018-09-22T14:00:30Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 22, 2018, 5:55pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/21 "2018-09-22T17:55:43Z")
> [@scr4](#): > > **beowulff** is probably referring to solar car competition vehicles [like this](https://en.wikipedia.org/wiki/World_Solar_Challenge#/media/File:Solar_Car_Tokai_Challenger.JPG). Yep. ---
**Author:** ![glowacks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/glowacks/32/5548_2.png) [@glowacks](https://boards.straightdope.com/u/glowacks)\ **Post date:** [September 22, 2018, 9:24pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/22 "2018-09-22T21:24:14Z")
> [@scr4](#): > > I found [this article](https://mashable.com/2016/06/21/toyota-prius-phv-solar-panels/#rCzpfcVjVOqV) that says the Prius solar roof didn’t pass the US safety standards (namely the rollover crash test) and therefore wasn’t offered in the US. I don’t understand how a solar panel is any less safe than sunroofs though. Raising the center of gravity of the car by putting more weight in the roof increases the rate of rollover accidents. Solar Panels are heavy. Glass for sunroofs is not. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 22, 2018, 10:16pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/23 "2018-09-22T22:16:35Z")
> [@Stranger\_On\_A\_Train](#): > > …unless you need the power to trickle charge a battery or run a low power microcontroller. Yea, for $20 a person can get a [solar-powered trickle charger for their car](https://www.amazon.com/dp/B015ZGDYII/). It’s a good solution for a vehicle that’s not driven for extended periods of time. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [September 22, 2018, 11:25pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/24 "2018-09-22T23:25:17Z")
> [@beowulff](#): > > Multiply that by the solar panel efficiency (20%, best case), Nitpick: 20% is not the best efficiency for solar panels. It’s typical of the efficiency of commecially available single-junction panels. You can do better with multi-junction panels. Typical efficiencies for those are 30 to 35 % for commercially available ones, and specially built ones (such as those put on space probes) are over 40%. They’re much more expensive than single-junction, but if you have limits on space and/or mass, they’re the way to go. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 22, 2018, 11:31pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/25 "2018-09-22T23:31:44Z")
> [@dtilque](#): > > Nitpick: 20% is not the best efficiency for solar panels. It’s typical of the efficiency of commecially available single-junction panels. You can do better with multi-junction panels. Typical efficiencies for those are 30 to 35 % for commercially available ones, and specially built ones (such as those put on space probes) are over 40%. They’re much more expensive than single-junction, but if you have limits on space and/or mass, they’re the way to go. Yeah, there are all kinds of hyper-efficient cells. You just can’t buy them. 20% efficiency is generous for a production panel, although there are a few with a smidge more points. See here: [How Efficient Are Solar Panels? Top Brands Compared in 2024 | EnergySage](https://news.energysage.com/what-are-the-most-efficient-solar-panels-on-the-market/) ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [September 23, 2018, 3:32am UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/26 "2018-09-23T03:32:46Z")
> [@beowulff](#): > > Yeah, there are all kinds of hyper-efficient cells. You just can’t buy them. > 20% efficiency is generous for a production panel, although there are a few with a smidge more points. > See here: [How Efficient Are Solar Panels? Top Brands Compared in 2024 | EnergySage](https://news.energysage.com/what-are-the-most-efficient-solar-panels-on-the-market/) I think I got confused between what individual cells can produce and what manufacturers can produce in panels. Apparently, there’s a big difference. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [September 23, 2018, 4:08am UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/27 "2018-09-23T04:08:20Z")
> [@glowacks](#): > > Raising the center of gravity of the car by putting more weight in the roof increases the rate of rollover accidents. Solar Panels are heavy. Glass for sunroofs is not. The weight of a solar panel /is/ the glass. Solar panels are heavy because they are big pieces of glass. A solar-panel-coating of a car roof wouldn’t add much weight, but would be difficult to apply, and without a good protective surface would last only minutes in high speed traffic. That is as much an economic problem as a technical one: You can’t use anything even vaguely like an off-the-shelf panel. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [September 23, 2018, 12:52pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/28 "2018-09-23T12:52:07Z")
Solar trickle chargers and such are a waste for quite a few people. For example, us. We park our cars in the garage. And if we were to park in the driveway there’s a big ole maple shading almost all of it. While we do make a few short trips here and there and the car will be out in the Sun some, the amount of charge added is going to be insignificant compared to the charge added by the alternator during the trip. We rarely go somewhere and leave a car parked all day. And then there’s places like Seattle and Portland for large stretches of the year. ---
**Author:** ![jjakucyk](https://avatars.discourse-cdn.com/v4/letter/j/3d9bf3/32.png) [@jjakucyk](https://boards.straightdope.com/u/jjakucyk)\ **Post date:** [September 23, 2018, 2:07pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/29 "2018-09-23T14:07:05Z")
> [@Melbourne](#): > > The weight of a solar panel /is/ the glass. Solar panels are heavy because they are big pieces of glass. A solar-panel-coating of a car roof wouldn’t add much weight, but would be difficult to apply, and without a good protective surface would last only minutes in high speed traffic. > > That is as much an economic problem as a technical one: You can’t use anything even vaguely like an off-the-shelf panel. Doesn’t seem to be a problem for the massive sunroofs on a lot of cars. It’s not like this would be some off-the-shelf component anyway. No auto glass is. It would be contoured and integrated into the roof, not something bolted on top. > [@ftg](#): > > Solar trickle chargers and such are a waste for quite a few people. > > For example, us. So what? There’s lots of features in all sorts of products, whether cars, TVs, computers, phones, washing machines, etc., that aren’t used by everyone, and some can be fairly expensive. In this case there’s benefits to panels that can work while driving too, instead of just when parked. Because something might not work for you it doesn’t mean it’s useless. You may even be a small minority. > [@ftg](#): > > And then there’s places like Seattle and Portland for large stretches of the year. Germany has the most solar power generation in the world despite being quite far north and cloudy, even worse than the Pacific northwest. [https://upload.wikimedia.org/wikipedia/commons/9/9d/SolarGIS-Solar-map-World-map-en.png](https://upload.wikimedia.org/wikipedia/commons/9/9d/SolarGIS-Solar-map-World-map-en.png) And just because something may not work well in one place that doesn’t mean it’s not good somewhere else. ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 23, 2018, 6:09pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/30 "2018-09-23T18:09:01Z")
[Lightyear One](https://lightyear.one/lightyear-one/) is planning to sell a practical electric car that can get an appreciable fraction of the required traction energy from sunlight in a sunny climate. That is, an electric car that doesn’t have to be charged as often (or as long) because part of the electricity for driving comes from its solar panels. Of course, as mentioned, the [World Solar Challenge](https://en.wikipedia.org/wiki/World_Solar_Challenge#Rule_evolution) competition already had solar cars in 2005 that could cross the entire Australian continent at more than 100kph on solar energy alone. Although these were cigar-shaped, one-person cars with no air conditioning. In 2017, the winner of that competition in the Cruiser class was a four-seat “practical” solar car that indeed transported four persons for most of the race. About half of the electricity (AIUI) it used came from its solar panels, about half was from overnight charging. ---
**Author:** ![Wesley\_Clark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wesley_clark/32/20581_2.png) [@Wesley\_Clark](https://boards.straightdope.com/u/Wesley_Clark)\ **Post date:** [September 23, 2018, 6:54pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/31 "2018-09-23T18:54:58Z")
> [@beowulff](#): > > The impediment is the amount of energy available in sunlight - 1KW/square meter. Multiply that by the solar panel efficiency (20%, best case), and battery and inverter efficiency (90%, best case), and allow for all the other losses, and you get around 175W of electrical power per square meter. > The average car roof is, what? 4’ x 6’ in area? Let’s be generous and say that’s 2 square meters. That gives you 350W of power in direct sunlight. That’s enough for a very efficient electric car, but not nearly enough to do anything very useful with a real passenger car. > The Tesla S has a 100KWH battery pack. This solar roof would be able to charge that battery about 3% with 8 hours of direct sunlight. And, I’m being generous. My impression is an electric car can travel about 2-4 miles on 1 kwh of battery. So if you’re only getting 350W over maybe 4 hours, thats less than 5 miles a day of travel. ---
**Author:** ![SamuelA](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@SamuelA](https://boards.straightdope.com/u/SamuelA)\ **Post date:** [September 23, 2018, 6:59pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/32 "2018-09-23T18:59:59Z")
> [@Wesley\_Clark](#): > > My impression is an electric car can travel about 2-4 miles on 1 kwh of battery. So if you’re only getting 350W over maybe 4 hours, thats less than 5 miles a day of travel. Tesla model 3 is 241 watt hours/mile. Source is [here](https://forums.tesla.com/en_CN/node/124359). That’s an actually existing, semi-mass produced electric car that is efficient. So if you only get 4 sun-hours per day in your location (typical for the south) and have 300 watts of panels exposed at any given moment (the problem with putting the panels along a car’s aeroshell is that no matter how you do it some will be shaded and the angle will not be optimal), you gain 5 miles per day. Which isn’t nothing, and this additional charging protects the car’s traction battery if it was low when you parked. But no, this probably won’t be routinely done. ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 23, 2018, 7:17pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/33 "2018-09-23T19:17:37Z")
The Lightyear One plans to use approx. five square meters of solar cells. That’s a lot more than you can fit on a normal car, but this car will be designed around that requirement. If they manage to get that, and in an optimal environment (sunny day, optimal orientation of the car) they seem to plan on getting some 10kWh out of those cells on the best days. (I get \>30kWh out of the ~30 sq.m. of panels on my roof, but that’s way op north in a dark climate, not optimally oriented, and not the most modern efficient cells either.) They also try to get the energy consumption down to 9kWh/100km (also under optimistic assumptions). Note that Bjørn Nyland already gets around 13kWh/100km driving 90kph on the highway in a Hyundai Ioniq -type regular electric car. It seems realistic to get that down with more radical aerodynamics, lighter weight and more optimistic assumptions. So they hope to get a little over 100km of driving per day, purely with the solar cells on the car. (You charge it normally to drive further.) ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 23, 2018, 7:28pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/34 "2018-09-23T19:28:39Z")
Reading more closely, they’re actually claiming closer to about 80km of driving range per day on the car’s solar panels alone (in the summer in Portugal). They only need to generate about 7kWh of solar electricity per day for that. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [September 23, 2018, 8:49pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/35 "2018-09-23T20:49:51Z")
Anyone know something about [“smart windows”](http://www.sciencemag.org/news/2018/06/skyscrapers-could-soon-generate-their-own-power-thanks-see-through-solar-cells)? The idea is that glass can be designed with materials that filter UV and/or IR leaving most of the visible light alone. The amount of power generated is fairly small, but if the material is produced en masses for the construction industry (as that article speculates might occur, as it can be adjusted to on-demand so its AC cost savings could be substantial) cost could be reasonable. Integrated into a glass roof, like much of the Tesla 3’s, won’t get the car very far, but it could still be of some use. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [September 23, 2018, 9:11pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/36 "2018-09-23T21:11:24Z")
Yeah, not enough energy density there to fully power anything more than the covered bicycles that have those occasional Tour de Sol cross-country derbies. [Solectria](https://en.wikipedia.org/wiki/Solectria_Force) first tried it over twenty years ago. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [September 23, 2018, 9:32pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/37 "2018-09-23T21:32:45Z")
From reading this thread, it appears that a solar-paneled car roof will make sense if the photo-voltaic efficiency improves to about 10 times what it is today. I hope that can happen, and I hope I live that long. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [September 23, 2018, 10:01pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/38 "2018-09-23T22:01:42Z")
> [@Musicat](#): > > From reading this thread, it appears that a solar-paneled car roof will make sense if the photo-voltaic efficiency improves to about 10 times what it is today. I hope that can happen, and I hope I live that long. That is a physical impossibilty. A half order of magnitude improvement, which is beyond a y rational expectation, would be miraculous. Stranger ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [September 23, 2018, 10:08pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/39 "2018-09-23T22:08:56Z")
> [@Musicat](#): > > From reading this thread, it appears that a solar-paneled car roof will make sense if the photo-voltaic efficiency improves to about 10 times what it is today. I hope that can happen, and I hope I live that long. Ten times 20% is more than 100%, so that’s not actually possible. But [This report](https://www.sciencedaily.com/releases/2018/09/180910093533.htm) gives some hope that we can get close to 100%. It doesn’t look like they’ve made an actual solar cell with this tech yet, so it’s several years away from prime time at the least. ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 23, 2018, 10:25pm UTC](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731/40 "2018-09-23T22:25:25Z")
Well I still think you are all wrong. A car that travels quite long distances on its own solar panels exists today and it is not a covered bicycle. It is the [Stella Lux](https://solarteameindhoven.nl/stella-vie/stella-lux/) which won the 2017 race. It generates 7-10kWh per day (my estimate) from its own 5sqm of solar panels and it travels 100km on about 4kWh (also my estimate). All of that is today, under favorable circumstances. That’s what they are trying to turn into the Lightyear One. [Previous page](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731.md?page=1) [Next page](https://boards.straightdope.com/t/is-it-feasible-to-put-solar-panels-on-the-roofs-of-cars/821731.md?page=3)