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.

A photo of an electric car plugged into a charging station.
Putting a little juice into my car.

I bought an electric car this year, and it’s a been a great experience.

That’s not just because the car itself (a Hyundai Ioniq 5) is great - which it is. Because I’ve got solar panels, I know that my driving is being at least partially offset with renewable energy. (Even before I bought the car, my solar array was supplying only about 85% of my household needs - a result of limited roof space.)

It turns out that I’m not alone: a large percentage of solar households either already own an EV, or are considering it.

If you own an electric car or you’re thinking of getting one, a home solar photovoltaic system is a great pairing. You can literally refuel your car with sunshine. How cool is that?

You might have a lot of questions about how solar and electric cars work together. Having owned my EV for several months now (and having been a solar homeowner since 2013) I’ve got a few lessons to pass along. Bottom line: it’s less complicated than you probably think.

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 the car is sitting in the sun?

Seems like a good idea, right? Get free fuel while you drive, maybe even never have to fill up!

There’s actually one popular production car that does - or rather, it did for a limited edition. The 2022 Hyundai Ioniq 5 was available with about 200 watts of solar photovoltaics in the roof. Hyundai claims that’s good enough to add about 3 miles of range per day if parked in a sunny location. That’s better than nothing, but I’m not sure 3 miles of “free” range a day is really worth the additional cost and complexity of the solar roof.

How about a completely solar powered car? 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:

Lineup of some vehicles participating in the Dell-Winston School Solar Car Challenge. Credit: JPL NASA
A lineup of vehicles participating Solar Car Challenge. (JPL NASA)

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.

Question: how I charge my car only with solar electricity?

This is a question I’ve come across quite often. If you have solar panels, it’s natural that you might want to charge your car directly with solar electricity - that is, getting the electricity that you generated with your solar panels and sticking it directly into your car.

If your solar array is grid-connected - as the vast majority of solar homes are - the answer is easy: don’t worry about it! This is true whether your reasons are financial or environmental.

If you’re trying to save as much money as possible, then you want to pay attention to billing scheme with your utility company. This means understanding whether you have net metering or net billing, and whether you have a time-of-use plan.

Environmentalists shouldn’t worry about charging directly with solar power either. The reason is that the electric grid produces varying amounts of pollution at different times of day, and it’s usually worst in the middle of the day when “peaker” power plants fire up to meet high demand.

Peaker power plants are often older, dirtier, and less efficient. There’s more than a thousand of them around the United States.

Peakers are often kept on standby and might be fired up only during really high demand, such as hot summer days when air conditioning use is at its highest.

For example, there’s <a href=“https://www.utilitydive.com/news/campaign-to-shut-down-new-york-citys-peaker-plants-gains-congressional-all/605670/"rel="nofollow noreferrer”>19 of them in New York City and some of them are more than 60 years old and powered by fuel oil or kerosene. They’re kept around and might be used only a few hours a year to meet electricity demand on the hottest summer days - which happens to be the time when solar panels are producing electricity.

There’s a proposal to replace New York’s peakers with wind power, solar, and batteries. The bottom line is that if you want your solar panels to have the maximum environmental benefit, it’s generally best to charge your electric vehicle during off-peak hours and let your solar panels dump their electricity into the electric grid so they can help reduce the load on peaker plants.

The situation is a little more complicated in places with lots of solar, such as California. There’s sometimes so much solar electricity that it’s better for the environment if you charge your EV around noon. You can read my article on the duck curve to learn about this phenomenon.

Things are also more complicated if you don’t have full net metering. The next section below explains the relationship between net metering, net billing, and time-of-use plans.

What’s the best time to charge your electric car?

A lot of solar homeowners have time-of-use (TOU) plans with their electric utility company. (It’s actually a requirement in California.) If you have TOU, your price of electricity changes during the day and from winter to summer. I wrote a guide on TOU that explains it indepth.

It’s also good to understand the basics of net metering, if you’re not familiar with it already. Net metering means you get a full credit for any unused solar electricity you sell back to the utility. Unfortunately, not all utilities offer it. You can read our guide on net metering to learn more.

Once you have a grasp of TOU and net metering, you’ll be able to make sure that you’re charging your EV when it maximizes the financial return and environmental benefit of your solar panels.

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, it’s better to use electricity during the times that your panels are generating 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.

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 billing

If you’re a solar homeowner and currently don’t have a time-of-use rate, it would be a good idea to see if it’s an option.

For example, my utility company in New York is National Grid, and they offer an optional TOU plan aimed specifically at EV owners. I opted into it recently after purchasing my EV. Under this plan, there is a “super-peak” rate during the summer between 2 p.m. and 6 p.m. While the super-peak rate is much more expensive, the upside is that my net metering credits follow the TOU rate. This means that if I’m not using much electricity during super-peak hours and let my solar panels dump their electricity into the grid, I’ll get a credit at the much higher super-peak rate.

The hours between 11 p.m. and 7 a.m. are my off-peak rate. This is when electricity is cheapest, so that’s when I charge my EV.

New solar homes in California are required to have a TOU rate, but in other places (like here in New York) it’s optional, so check with your utility company to see if it’s available. Be sure to also ask if your net metering will follow the TOU rate. If it does and a significant amount of your electricity usage is EV charging, it’s quite likely that TOU would be beneficial for you.

Cheatsheet on the best time to charge your EV

Once you understand what your net metering and TOU arrangement is, you can figure out when the best time to charge your electric vehicle is. This cheatsheet will help you make the right decision:

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.

Most new electric cars have a setting that allows you to schedule the charging time, making it easy to charge your vehicle with cheaper off-peak electricity.

If your car doesn’t have this feature, you can buy an electric car charger with a built-in timer. You can read more about chargers later in this article.

How many solar panels do I need to charge my Tesla (or other electric vehicle)?

The number of solar panels you need to charge your vehicle will depend on how much you drive, the vehicle’s fuel efficiency, and the productivity of your solar array.

You can find out the fuel economy of your car by looking it up at fueleconomy.gov, or referring to the fuel economy label that was provided with the car when it was new.

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.

Sample label from fueleconomy.gov for an electric vehicle.
Sample label from fueleconomy.gov for an electric vehicle.

The energy contained in one gallon of gasoline is equal to 33.70 kilowatt-hours (kWh) of electricity. That’s an interesting bit of trivia, but it’s not very practical because your monthly utility bill is based on kWh. The fuel economy label helpfully lists the number of kWh of electricity needed to travel a distance of 100 miles, which is a more useful figure. (This is highlighted above with the red circle.)

As an example, let’s say that you’ve got a Telsa Model 3 Standard Range. It consumes 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 more 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, but a reasonable estimate of the losses is 15%. That means 15% of the electricity we use to charge our EV is lost as heat.

This means that the final equation to calculate your electricity requirements for driving is:

(fuel economy in kWh per 100 miles) ÷ 100 miles × miles driven per day x 1.15

If we drive our Tesla 38 miles per day, the math is:

26 kWh ÷ 100 miles × 38 miles × 1.15 = 11.36 kWh
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. That's 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 makes this easy to determine.

Let’s pretend you live in Beverley Hills, California in Zip Code 90210, 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:

Sample result from The Solar Nerd calculator.
Sample result from The Solar Nerd calculator.

It tells you that you’ll need a 2.26 kW solar panel system. 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, so try the calculator yourself to get a quick estimate (or contact a solar installer near you for a precise estimate).

How many solar panels you’ll need depends on how much sun you get

The exact number of solar panels you’ll need depends on how sunny your location is.

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
Seattle8 - 13
Kansas City6 - 10
Miami6 - 9
Phoenix5 - 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.

What type of electric car charger do I need?

There isn’t any special equipment a solar homeowner needs to charge their EV. Just pick a charger that suits your needs best. Many people get by fine with the Level 1 “trickle charger” that comes with their vehicle. It all depends on how much you drive.

I’m not an expert on EV charging equipment, but here’s a few basics I’ve learned that might be helpful to you.

First of all, what is commonly called a charger is technically known as an electric vehicle supply equipment, or EVSE. It’s basically a sophisticated switch that lets you charge your car safely. It doesn’t transform the voltage or do AC-to-DC conversion: all of that is handled by equipment within the car.

A Level 1 EVSE operates at 120 volts. It’s the slowest type of charger available, but it works with common household outlets. The typical 3-prong household outlet is known as a NEMA 5-15. You can safely use it to charge at 12 amps. Another common 120V outlet is the NEMA 5-20. It looks similar, but the left slot has a t-shape. It’s paired with a 20 amp circuit breaker and can handle 16A of continuous current.

There’s one more type of 120V outlet which is somewhat common in North America, and it’s the TT-30. It’s mostly used for RV applications (the TT is short of travel trailer) and can handle 24A of continuous current. I actually have one of these in my garage and it’s what I use to charge my Ioniq 5. One important caveat however: some EVs have a limit on how fast they can charge at 120V, and mine tops out at about 17A - far short of what the TT-30 outlet can deliver.

A Level 2 EVSE operates at 240 volts and can charge your car about 4 times faster than a Level 1 device. Many homes might a 240 volt outlet in their laundry room or in their kitchen for an electric stove, but they’re less common in a garage.

I don’t have a Level 2 EVSE because I have a detached garage, and an electrician quoted me $7,000(!) to run a 240V new service out there. Fortunately, I’m able to get by with Level 1 charging.

If you don’t have a very long commute, you might be able to stick with L1 charging too, otherwise you’ll need to contact an electrician to add a 240V line to your parking spot. The other consideration is that you’ll want to do all of your charging according to the cheatsheet above, and an L1 charger might not be fast enough to fit all your charging within your time window.

(Pro-tip: many solar installers also install EVSEs, including those in the Solar Nerd network. One of them actually will throw one in for free if you do a solar installation with them!)

Apart from choosing between an L1 and L2 EVSE, there’s probably two main things to consider:

  • Pick a device that has an electrical safety certification, such as UL.
  • Pick one with a timer if your car doesn’t have a scheduled charging feature built-in.

UL listing means that it’s gone through safety certification. This is a concern with any device that is handling thousands of watts of electricity, and is especially a concern if you’re shopping for cheap EVSEs on Amazon.

Some high quality brands (note: Amazon sponsored links) with UL certification include Grizzl-e, ChargePoint, JuiceBox, and ClipperCreek. Lectron has a TUV safety listing (TÜV Rheinland is based in Germany).

Any of these products would be a good choice, and safer than the many other random brands you can find on Amazon.

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.

SolarEdge inverter with integrated EV charger
SolarEdge inverter with integrated EV charger.

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.

Enphase Microinverters

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 charging 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.

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#Electric Cars

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