It seems like even a small solar panel on the roof of an electric car would make a lot of sense. When you’re sitting at work for 8 hours, your car can just soak up some sun. I’m assuming there is some sort of problem related to cost, rather than efficiency, but I can’t figure it out.
Edit: Thanks for the help. It makes a lot of sense now. I appreciate the in-depth answers too.
In: Engineering
My understanding is that the energy gained is insignificant compared to the cost of engineering. This will change as the solar panel technology advances, especially with the clear panels bring developed.
Currently I believe it is similar to trying to save money on your water bill by filling your toilet tank with rain water. Even if you live somewhere that it rains a lot, it won’t be effective 100% of the time and you are going to need some creative installation to automate the process. The cost of water for most homeowners is low enough to not be a concern on this scale of savings.
Solar panels are fairly heavy and so installing one on a car will increase energy needs to propel the car. One that size would also take several days to charge the battery up to full. It is possible to implement them in a system to help extend battery life, but it could not replace charging them by plugging them in.
The cost/benefit analysis doesn’t hold up. You couldn’t put enough panels on the car to charge it in a reasonable time even in perfect conditions and the cost of adding the panels would raise the total cost of the car. So, could you? Sure. Would you alienate more buyers for virtually no extra value proposition? Absolutely.
It adds weight for minimal charging capability
A Tesla Model S is 5 meters long and 2 meters wide giving you 10 m^2 of area to work with. Best case you’ll get about 300 watts/m^2 in really good conditions so you can get about 3 kW from the solar panels on the car which can recharge that 90 kWh battery by about 3% per hour
But this assume covering the *entire* foot print of the car with solar panels in optimal conditions. Realistically you’re looking at 4 m^2 on the roof and less than 200 W/m^2 or about 800 W which recharges that 90 kWh battery at about 0.9% per hour.
The additional weight of the solar panels will burn through additional power every time the car goes to accelerate, and the added cost/complexity will make the car notably more expensive.
Solar panels and electric vehicles make a better combinations when you put them on an overhead structure with far more area that gets to stay in a single place
Solar panels can provide about 150 Watts per square meter in ideal sunny conditions. Let’s say we put a 1 square meter panel on the roof of the car to generate 150 Watts.
Your 150 Watt solar panel sitting in the sun for 8 hours will generate 1200 watt hours.
An EV battery capacity is between 18 kilowatts hours on smaller EVs up to 100 kilowatt hours for the long range Tesla. A kilowatt is 1000 Watts. The 1.2 kilowatt hours from the panel can only charge 6.7% of the smaller 18 kilowatt hour battery or 1.2% of the larger 100 kilowatt hour battery. This is a very small percentage for the costs of adding the panels.
Solar panels are heavy and do not generate very much power per area.
A typical car has perhaps 4 m^2 of area on top of it. In the very sunniest areas, that’s about 20 kWh of energy collected per day on average. A Tesla gets around 3 miles per kWh, so that adds about 60 miles of driving – about a 25% increase in range – at most. But the solar panels are heavy (so they drag down the efficiency), they would require you to park in the sun (which you often won’t be, especially in cities where EVs are most common), most areas get less than that 20 kWh (that number is Arizona-desert-level sunlight), and they’re expensive.
Finally, if you wanted to add weight that badly, you’d just add a bigger battery and get more extension of your driving distance.
All in all, it’s easier to just plug in.
Not really. The additional weight just wouldn’t make sense given the benefit. Even for a fairly large car, it would be fairly difficult to get more than 2-3 sq meters of solar roof (unless it gets pretty crazy). Even with the most efficient panels (and drivers willing to park in the sun all day), this would be around 3-4kWH at best. A typical fully electric car has about 60-100KWH batteries. So it would make very little difference in terms of charging a car.
First, some do. Fisker has announced an SUV that has a solar panel on the roof that they claim will get you 1,000 miles of travel over the course of a year.
The reasons that most don’t come down to cost, complexity and marketing decisions. Having that solar panel on the roof adds costs to the car that might not ever be paid back over the lifespan of the vehicle. It also adds complexity to the charging mechanism which introduces more chances for failure. Finally, people like to buy cars with sunroofs and all glass tops.
With the amount of space available for placing solar panels (ignoring weight considerations), the power output of such panels could still take days to fully charge a vehicle, even with direct sunlight for most of the day.
Typical solar panels put out around 15 watts per square foot under ideal circumstances, and at best, a typical car might only have about 20 square feet or so (a 5×4 foot rectangle or equivalent) to work with.
As a reference, the Tesla Model 3 has a minimum battery capacity of 50,000 watt hours, which translates to ~3333 [sq. Ft * hours] to fully charge (~166 hours of optimal sunlight for a 20 sq ft panel)
Solar panels require a lot of space, more than you get on a car, to provide the amount of power required for a car.
There are actually some cars powered just by on-board solar panels, but they’re basically just engineering exercises; they run on bicycle wheels and have ultra-aerodynamic bodies and only hold one person practically lying down to keep the roofline that much lower.
Edit: Just to bring some math into this, a normal car cruising at highway speed uses about 15 kilowatts of power, which on a really good sunny day would take four or five cars’ worth of solar panels to generate.
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