The complete guide to charging your electric car with solar power
Solar power is a natural complement to an electric car. This guide will explain how it actually works and how to maximize the value of your home solar system.
If you own an electric car, or are thinking of getting one, a home solar photovoltaic system is a great pairing. Not only can your home and commute be emissions-free, but you can generate the “fuel” for your vehicle from your own rooftop. If energy independence and green energy are things you care about, this is a really attractive prospect.
This guide explains the details of how this system works, and answers some common questions that people have.
All good? Okay, read on!
List of currently available electric cars in 2019
The great thing is that there is an ever-expanding selection of excellent electric cars available in 2019, and even more coming in 2020.
Subpar electric “compliance” cars have been available for a few years now. A compliance car is one that is primarily made to meet the California Air Resource Board (CARB) mandate that manufacturers must sell a minimum number of zero emission vehicles. Compliance cars are electric vehicles that are built upon an existing car platform, which means that electric components such as batteries need to be crammed into a chassis that isn’t designed for it. An example of this is the Ford Focus Electric, which used most of the trunk space for batteries but still managed a range of only 76 miles.
Fortunately, after a few years of design work, manufacturers are finally starting to release cars that have been designed ground-up to be electric. For example, Volkswagen’s MEB platform is the underpinning of four VW electric vehicles that will be released by 2022, as well as other vehicles from VW subsidiaries Audi, Skoda, and SEAT.
Below is a partial list of electric vehicles and plugin hybrids that are available now. Note: some highly anticipated electric vehicles can still only be pre-ordered, or have very limited availability. Those cars, such as the Audi e-tron, aren’t listed.
- BMW i3
- Chevy Bolt
- Fiat 500e
- Hyundai Ioniq Electric
- Hyundai Kona EV
- Kia Niro EV
- Nissan Leaf
- Toyota Prius Prime
- Tesla Model 3
- Tesla Model S
- Tesla Model X
- VW e-Golf
Why don’t cars come with solar panels built-in?
Let’s get one common question out of the way: why don’t car manufacturers put solar panels on the roof of electric cars so that you can charge your battery while your drive?
Seems like a good idea, right? Get free fuel while you drive, maybe even never have to fill up!
Technically, it is possible. The Solar Car Challenge is a competition for students that’s been running since 1995. The vehicles race from Texas to California, powered only by electricity obtained from vehicle-mounted solar panels. The cars look like this:
But these cars can travel across country only because they’re extremely light, streamlined, have no crash protection, and carry one passenger. A far cry from your family SUV.
Even if you can’t completely power your car with solar panels, could you come close, or at least generate enough power to make it worthwhile? Let’s take a look.
The most efficient solar panels these days generate about 400 watts of power, and the average car roof is about the size of one panel. If you read our guide to solar panel specifications, you’ll know that a solar panel generates less than its nameplate rating in real-world conditions. But for simplicity, let’s say that we actually get 400 watts out of our roof.
The least expensive Tesla, the Model 3, has a 211 kilowatt motor. That means, with the pedal to the metal, the motor consumes 211,000 watts of power. And this is not Tesla’s performance model - it’s the most basic vehicle in their lineup.
211,000 watts is about 527 times more energy than our solar panel can generate. This means if your Tesla had to be powered directly by one solar panel, it could only muster 1/527th of its maximum power.
What about a less powerful electric car? The Kia Niro EV is a new electric car that’s more budget-oriented. It has a 150 kilowatt motor. That’s still 375 times more power than our one panel can output.
So far, it’s not looking very practical. How about if we’re very patient, and just want to use the solar panel to charge the battery while the car is parked? Well, the Niro EV has a 64 kWh battery. It would take our 400 watt panel 160 hours in full sun to charge it from empty.
Lightyear One and Sono MotorsBut what if you could make your car extremely efficient and cover every possible surface with solar panels? Could you make a practical passenger car work? That’s what two startups are now trying to do now.
Lightyear One is a startup based in The Netherlands. Its car will seat 5, drive 450 miles on a fully charged battery, go from 0 to 60 in 10 seconds, and with built-in solar panels gain 7.5 miles of range per hour on a sunny day. Here’s what it looks like:
If you look closely, you can see there’s no rear window. Instead, the area where the rear window would be is taken up by solar panels. There’s also solar panels on the roof and hood of the vehicle. The specs don’t state how much power they generate, but I would guess that the car probably generates in the neighborhood of one kilowatt from its solar panels in bright sunlight.
Sono Motors is the other company that’s attempting to bring a solar car to market. Based in Germany, the approach Sono is to develop a more conventional-looking vehicle that has proportions similar to a Smart car and other micro cars.
The vehicle is called the Sion. It will seat 5, have 155 miles of range, hit a top speed of 87 mph, and packs a 35 kWh battery.
As you can see, the Sion does have a small rear window, unlike the Lightyear One. Every available surface is covered with solar panels, including the roof, hood, rear hatch, and doors. Sono says that altogether the vehicle has 1.2 kW of monocrystalline solar panels that will add 21 miles of range per day in sunny weather.
Prototypes taking pre-orders now
These extreme designs to maximize solar energy collection and radically reduce aerodynamic drag are what’s required to make a solar vehicle feasible.
It’ll be interesting to see if either company can get their vehicles out of prototype and onto the market. Both companies are currently taking pre-orders.
How do you use solar panels to charge an electric car?
Now that we’ve established that the practical way to charge an EV with solar electricity is with panels on your home and not your car, let’s do a super quick overview of how a home photovoltaic system works.
On your roof (or, more rarely, on scaffolding in your backyard) you have racks that your solar panels are fixed to. Wiring on the back of the solar panels connect to inverters that turn the DC power from your panels into AC power that your home uses. Those inverters send AC power into your home where it gets used by your coffee maker, TV, lights, and all your other appliances.
If your panels generate more electricity than you use, the excess gets dumped into the grid, where it gets used by your neighbors.
Charging your car only with solar electricity
A lot of people who have an electric car and solar panels want to charge their cars using as much solar electricity as possible, rather than grid power. It’s understandable: after all, anyone who has solar panels and an electric car probably cares a lot about the environmental benefits of renewable energy.
But, as you’ll read next, it doesn’t matter much either from an economic or environmental point of view whether your car is charged directly by your panels. Instead, your charging schedule should depend on whether you have net metering and a time-of-use plan with your utility.
Net metering vs net billing
If you have net billing, you don’t receive a full credit for any excess solar electricity that you send into the grid. Rather than being credited at the retail rate of electricity, you are often only paid the wholesale rate of electricity, which is the rate that power plants charge your utility company for electricity. Under net billing, you’re incentivized to use your solar electricity rather than grid power.
Unfortunately, this means using electricity during the day, rather than morning or evening. But most people with EVs charge their cars in the evening when they get home from work, which means that having net billing is not an ideal situation for an EV owner. If this is your case, one possibility is to minimize your charging during the week and instead try to make it to the weekend so that you can be home to charge your car during daylight hours. However, this may mean stretching the range limit of your car, which can be anxiety-inducing.
A much better situation is to have net metering. If you are under net metering, you get full credit for energy you send into the grid. This eliminates the urgency to use your own solar electricity.
Time of use billingBeside the issue of whether you have net billing or net metering, another consideration is whether you have time of use billing (TOU) with your utility. If you do have TOU, it means that you have peak hour rates during which you pay more for electricity, and off-peak hours during which your electricity costs less.
TOU is an important factor in deciding the optimal time to charge your EV, but you still have to balance that against whether you have net billing or net metering. Here’s a table that summarizes when you should charge for each of the possible scenarios:
|Scenario||Best time to charge|
|Net metering with TOU||Off-peak|
|Net metering without TOU||Anytime, but off-peak is best to minimize carbon intensity.|
|Net billing with TOU||Charge during off-peak hours if the off-peak savings is greater than the difference between your retail rate and the net billing (avoided cost) rate. Otherwise, charge when your solar panels are producing power.|
|Net billing without TOU||Charge when your solar panels are producing power.|
Finally, even if you don’t have time-of-use billing, if you care about the carbon intensity of the electricity you use, you should still pay attention to the time of day that you charge your car. This is because different power plants supply electricity at different times of day. Electricity demand during peak hours tends to be met by fossil fuel plants - often natural gas peaker plants - while off-peak electricity is often supplied by a higher proportion of carbon-free generators, such as hydroelectric, nuclear, and wind power plants.
You can read more about this in our article that explains the benefits of going solar. In general, carbon intensity is lowest at night after “primetime” TV is over. For a more accurate assessment of your electric utility’s carbon intensity in real time, visit WattTime, which is a non-profit that aggregates this data across the United States.
Electric cars allow you to schedule your charging time either from the dashboard or a smartphone application, giving you a way to automatically turn charging on only during off-peak times.
How many solar panels do I need to charge my Tesla (or other electric vehicle)?
There’s two ways of looking at this question:
- You want to offset your electricity usage with solar power.
- You want to charge your electric car in real-time with solar power, and never draw electricity from the grid.
The first option is a lot easier to achieve and, to be honest, is really the only one that makes sense for the homeowner as you’ll see from the calculations below. But if you’re hardcore, or just curious about what it would take to only ever charge your car off the grid, I’ll also look at the math behind going with option two.
Option 1: Offset your electric car charging with solar power
With this approach, you’re simply concerned about generating enough electricity to offset your daily electricity use from charging your car.
If you work a typical work day, this means that your solar panels will generate an excess of electricity during the day that will be sent into the grid.
When you come home in the evening, you’ll plug your car in for charging. If you’re on a time-of-use plan, you might configure your charging time so that it starts late at night when off-peak hours begin. Whatever the case, most or all of the electricity to charge your car will come from the grid.
Your goal in this case is to generate enough solar electricity to equal the electricity that your car takes from the grid for charging.
Knowing how much electricity in kilowatt hours that equals is a simple calculation. You need to know:
- The fuel economy of your car, which is listed on the fueleconomy.gov label as kW hours per 100 miles.
- The distance that you drive every day in miles.
The label below is the fuel economy label for an electric car. The main thing to notice is that the fuel economy is listed as MPGe, or miles per gallon equivalent. For an electric car, this is the distance that the car can travel on the energy equivalent to one gallon of gasoline.
The energy contained in one gallon of gasoline is equal to 33.70 kWh of electricity. But you’ll also notice that the label helpfully lists the number of kWh of electricity needed to travel a distance of 100 miles. (This is highlighted with the red circle.)
If you’re not sure of the fuel economy of your car, you can look it up at fueleconomy.gov.
For an example, let’s say that you’ve got a Telsa Model 3 Standard Range. It gets 26 kWh per 100 miles.
How much do you drive? Let’s say it’s 14,000 miles per year, which works out to 38 miles per day.
There’s one last thing to take into account, which is the fact that charging your car isn’t 100% efficient. Some electricity is lost in the conversion from AC to DC power, and in charging losses in the battery. It’s hard to get accurate figures on this for every electric vehicle out there, but a reasonable estimate of the losses is 15%. That means we’ll need to add 15% to our result account for this.
To calculate the electricity needed, our final equation is:
If we drive our Tesla Model 3 38 miles per day, the math is:So that means if you drive about as much as the average American, you need to generate about 11.36 kWh of solar electricity per day to offset your driving. Not too bad at all. But how many solar panels is that?
Well, that depends on your roof, your climate, and the efficiency of your solar panels. Fortunately, The Solar Nerd calculator does this for you.
Let’s pretend you live in Beverley Hills, California (congratulations!) and you have a roof with a medium slope facing south with no shade.
The calculator asks for your monthly usage, so we’ll multiply 11.36 by 30 days, which equals 340 kWh per month.
When you plug those numbers in, you’ll get a result like the following:
It tells you that you’ll need a 2.26 kW solar panel system to keep up with charging your Telsa Model 3 for 14,000 miles of driving per year. This is equal to 6 to 9 solar panels, depending on the efficiency of the panels you choose.
Your result will differ depending on your location and roof orientation, but as you can see, this is pretty simple to figure out.
Example analysis for a Tesla Model 3 (mid-range)
Here’s an example for a Tesla Model 3 (mid-range). According to the EPA, it has a rating of 123 MPGe combined. This means that if you drive 14,000 miles per year, this Tesla would consume 3,836 kWh of electricity per year.
That works out to 320 kWh per month. If you plug that into the calculator and assume a south-facing solar array with no shade, here’s how many solar panels you would need to generate that much power in different cities:
|City||# of solar panels for 14,000 miles of yearly driving|
|Seattle||8 - 13|
|Kansas City||6 - 10|
|Miami||6 - 9|
|Phoenix||5 - 8|
As you can see, power generation depends a lot on how much sun you have in your location.
Of course, the number of panels we calculated doesn’t include the rest of your household usage. To get that, just add your monthly household electical usage to the number of kWh needed for your driving. That will give you the total number of panels you’ll need to both charge your Tesla and completely power your household.
Option 2 (hardcore!): Only charge your electric car with solar power (ie. off-the-grid)If you want to charge your car using only power from the sun, you’ll need to calculate how many solar panels of output that is. The exact power requirement will vary according to the type of charger you use and the capacity of your onboard charger.
If you charge using a regular household outlet, also known as level 1 charging, you will draw about 1,500 watts.
On the other hand, if you’ve installed a level 2 charger on a 240v circuit, the power draw will be a lot higher. The exact amount of electricity depends both on the size of the charger onboard your car and the capacity of your level 2 charger.
The table below lists the onboard charger specifications for a few popular electric vehicles, the size of their onboard charger, and the solar panel wattage required to fully meet the demand of that charger, with the assumtion that the panels will generate a maximum of 75% of their nameplate efficiency:
|Vehicle||Onboard charger||Solar panel watts required||# of 400w panels required|
|Toyota Prius Prime||3.3 kw||8,800 watts||22|
|Hyundai Ioniq, Nissan Leaf||6.6 kw||17,600 watts||44|
|Chevy Bolt, Hyundai Kona EV, VW e-Golf||7.2 kw||19,200 watts||48|
|BMW i3, Tesla Model 3||7.7 kw||20,533 watts||51|
|Tesla Model S/X||11.5 kw||30,667 watts||77|
You’re going to need a lot of solar panelsAs you can see, even a vehicle which isn’t fully electric, like the Prius Prime, can suck a lot of power. The very largest residential solar systems allowed by most utilities is 25 kilowatts, while the average home solar installation is less than 10 kw. This means you would need a very large photovoltaic system (and be in full sunlight) to generate enough electricity to meet the full demand of your EV charger. If you have a Tesla with a massive 11.5 kw onboard charger, you simply won’t be able to charge your car with solar power only.
You could also use a household solar battery, but...The final option you can take if you really want to charge your electric car off-the-grid is to purchase enough home solar batteries to store enough power for your daily driving. This would work, and it means that you don’t have to worry about having enough real-time power flow from your panels to handle the charging. But it will be expensive.
If you read the section titled “Option 1”, you’ll know that the average American drives about 14,000 miles a year. If you do the math in Option 1, for a Tesla Model 3, that equals about 13 kWh of power per day, which just so happens to be the capacity of a Tesla Powerwall.
But that will completely drain your Powerwall every day, leaving no spare battery capacity if you want to use it for the rest of your home. So you’ll need two Powerwalls, which will cost $14,100 according to Tesla.
Don’t bother with off-grid electric chargingAs you can see, trying to achieve off-grid electric car is really expensive and impractical, and doesn’t make any sense from either an environmental or economic perspective.
Best solar inverters for electric car owners
If you’re planning a home solar system that will (or might) be used to charge an electric car, there are a couple inverter choices that may be better than others.
SolarEdge EV Charging Inverter
SolarEdge makes a 2-in-1 solar inverter and level 2 EV charger combo. The package includes their HD Wave string inverter, as well as an integrated level 2 EV charger. On it’s own, the EV charger will supply as much as 7.6 kw of electricity to your car from the grid. However, if your solar panels are producing power, the inverter has a “boost” mode that will channel as much as 2 kw of solar power to the charger, for a maximum combined output of 9.6 kw. The exact amount of boost will depend on your current solar output.
The main benefit of this system is that it cleanly integrates both the inverter and a level 2 charger in the same package, minimizing the number of devices you need to mount on your garage wall. It also has a number of other features, such as a smartphone app that lets you schedule charging around your time-of-use billing.
EVs are expected to grow rapidly over the next few decades. Because you should expect your photovoltaic system to last at least 25 years, there’s a good chance that you will own a couple generations of electric cars during that time, or maybe replace a second gasoline car with another EV.
This means that your electric needs may grow in the future. If you want to meet that future demand by adding solar panels, you’ll want to factor that into your current system design. The most flexible system for allowing future solar system growth is to use microinverters. This is better than a string inverter, or even a string inverter with power optimizers, because microinverters do not require your solar panels to be wired in strings, or have a requirement that all your solar panels have a similar voltage.
With microinverters, you can simply add panels to the system, and not be concerned about whether your new panels are from a different manufacturer. Not having to worry about strings can also simplify the installation, especially if your new panels need to be installed on a new section of roof.
Bottom line: Don’t worry about whether your car is powered by solar electrons
If environmental impact is your goal, you really shouldn’t worry about whether your car is powered by “solar” electrons, or if you have to sometimes use grid power.
Instead, focus on the most economical time to charge. Schedule your car charing according to your net metering and time-of-use situation, and only after that worry about if you can charge during off-peak hours to minimize the environmental impact. As was discussed in the article about the benefits of going solar, any excess power your solar panels generate will tend to offset fossil fuel peaker plants, helping to reduce the carbon footprint of the electricity supply.
From an environmental point of view, this will have the largest impact on your carbon footprint than worrying about whether your car was charged by the grid or your solar panels.