How Much More Solar Do You Need for an Electric Car?
Electric vehicles are becoming one of the biggest factors we look at when sizing a residential solar system. A homeowner might have a perfectly sized solar system today, but adding an electric vehicle can completely change the amount of electricity that home consumes.
That is because an EV is not a small electrical load.
Depending on how much you drive and how often you charge at home, an electric vehicle can add approximately 3,000 to 6,000 kWh or more of electricity consumption per year. For some homes, that could represent a 25% or 30% increase in electricity usage. For a lower consumption home, an electric vehicle could nearly double the amount of electricity being used.
So, if you are considering solar and either already own an electric vehicle or plan on buying one in the future, the EV needs to be included in the solar calculation from the beginning.
The question is not simply: How many solar panels do I need for an EV? The better question is: How much electricity will my EV use every year and how much additional solar production do I need to cover it? Once we know that number, we can properly size the solar system and determine whether additional battery storage should be included as well.
How Much Electricity Does an Electric Vehicle Actually Use?
The amount of electricity an EV uses depends on several factors.
The biggest factors are:
- How many miles you drive every year
- The efficiency of your vehicle
- The size of the vehicle battery
- How often you charge at home
- Whether you have access to charging at work or other locations
- Whether most of your charging happens during the day or at night
For solar sizing purposes, we typically see an electric vehicle add somewhere around 3,000 to 4,000 kWh per year for a homeowner who drives a moderate amount. For someone who drives more and charges almost exclusively at home, that number can easily reach 5,000 to 6,000 kWh per year or more. That is a substantial amount of electricity.
Understanding the Size of an EV Battery
One of the easiest ways to understand how much power an EV can use is to look at the battery itself. Electric vehicle batteries vary significantly depending on the manufacturer and model. Some vehicles may have batteries around 60 kWh, while larger vehicles can approach or exceed 100 kWh.
For a simple example, let’s use a 75-kWh EV battery. If that battery were completely empty and you charged it to 100%, the vehicle would require approximately 75 kWh of stored energy, before accounting for charging losses. Obviously, most people are not arriving home with a completely dead battery every night. Let’s say you normally arrive home with approximately 60% remaining. That means you need to replace approximately 40% of the battery.
75 kWh × 40% = 30 kWh So that single charging session could require approximately 30 kWh of electricity. To put that into perspective, some smaller homes may only consume 20 to 30 kWh of electricity throughout an entire day. Your vehicle can become one of the largest electrical loads on the property.
How Quickly Can EV Charging Add Up?
Let’s continue using that 30 kWh charging example. Suppose you replace approximately 30 kWh of energy around 18 times per month. That would be: 30 kWh × 18 = 540 kWh per month.
Over 12 months: 540 kWh × 12 = 6,480 kWh per year. That is more than 6,000 kWh of additional annual electricity consumption just from the vehicle. Now imagine the home previously consumed 7,000 kWh per year. After purchasing the EV, total annual consumption could potentially increase to more than 13,000 kWh per year. The home’s electricity usage has almost doubled. Nothing changed with the solar system. Nothing necessarily changed with the appliances inside the home. The homeowner simply added a very large new electrical load. This is why EV charging has to be considered when designing solar.
An EV Can Use Half as Much Electricity as Your Entire Home
This is probably the biggest point homeowners need to understand. An electric vehicle can consume a surprisingly large percentage of the home’s total electricity. We’ve seen homes that consume approximately 6,000 to 8,000 kWh annually before adding an EV. Then the homeowner purchases an electric vehicle, starts driving regularly, and charges almost exclusively at home. If that vehicle adds another 4,000 to 6,000 kWh per year, the EV can consume nearly as much electricity as the rest of the house. That drastically changes the solar calculation. This is also why we always ask homeowners about future plans when designing their solar system.
If you think you’re going to purchase an EV in the next 1 or 2 years, we want to know about it. It may make more sense to account for that additional electricity consumption now rather than installing a solar system that becomes undersized as soon as you purchase the vehicle.
How Much Additional Solar Do You Need for an EV?
Once we estimate the annual electricity consumption of the vehicle, we can determine approximately how much additional solar production will be required. Let’s use an example production yield of approximately 1,550 kWh per year for every 1 kW of installed solar. Actual production will depend on the property, roof orientation, shade, system design, location, and other factors, but this gives us a good example.
- If your EV adds approximately 3,000 kWh per year:
- 3,000 ÷ 1,550 = approximately 1.9 kW of additional solar
- If your EV adds approximately 4,000 kWh per year:
- 4,000 ÷ 1,550 = approximately 2.6 kW of additional solar
- If your EV adds approximately 5,000 kWh per year:
- 5,000 ÷ 1,550 = approximately 3.2 kW of additional solar
- If your EV adds approximately 6,000 kWh per year:
- 6,000 ÷ 1,550 = approximately 3.9 kW of additional solar
Now we can translate that into solar panels.
How Many Additional Solar Panels Does an EV Need?
Modern residential solar panels are much more powerful than they were several years ago.
Let’s use a 450-watt solar panel as an example. If we needed approximately 3.6 kW of additional solar: 3,600 watts ÷ 450 watts = 8 solar panels.
So, depending on how much the vehicle is driven and charged at home, an EV could potentially require somewhere around 5 to 9 additional modern solar panels. But this is only a general range. You cannot accurately say every EV requires 6 panels, 8 panels, or any other fixed number. The number of panels depends on how much electricity the vehicle actually uses and how much electricity each solar panel can produce at that particular home.
Your Roof Makes a Big Difference
Two homeowners could purchase the exact same electric vehicle, drive the same number of miles, and still require a different number of solar panels. Why? Because their roofs may produce differently. A roof with excellent sun exposure and very little shade may produce significantly more electricity per panel than a roof with partial shade. Orientation matters as well.
South, west, and east facing roof sections can all be good locations for solar, but they will have different production characteristics throughout the day. A ground mounted system may also have a different production yield because we have more flexibility over the orientation and tilt of the panels. This is why solar should always be sized based on production, not simply panel count. We first determine how many additional kWh the EV will require. Then we determine how much solar needs to be installed at that specific property to generate those additional kWh.
Solar Panels Are Only Half of the EV Equation
Once we determine how much additional solar you need, there is another very important question: When are you going to charge your vehicle? This becomes extremely important when battery storage is involved. If you are home during the day and can charge the vehicle while your solar panels are producing, that can be a very efficient way to charge. Your solar system is generating electricity, and the EV is consuming it at approximately the same time.
But that is not how everyone lives. A lot of homeowners leave for work in the morning and come home around 5 PM or 6 PM. They plug their EV in when they get home. At that point, solar production may already be dropping significantly. During certain parts of the year, the solar system may barely be producing at all by the time the vehicle starts charging. Now the electricity has to come from somewhere else. If the goal is to minimize how much power you purchase from the grid, that electricity may need to come from your home battery system.
EV Charging: One Battery Charging with Another Battery
This is probably the easiest way to understand EV charging with a home battery. Your electric vehicle is essentially a large battery sitting in your garage. Let’s go back to our example. Your EV has a 75-kWh battery. You arrive home and need to replace approximately 40%. That means the vehicle needs approximately 30 kWh. Now compare that with a common home battery. A Tesla Powerwall has approximately 13.5 kWh of usable storage. So, the 30 kWh you want to put into your vehicle represents more energy than two full Powerwalls can store. That does not mean you automatically need to install two additional batteries because you purchased an EV. But it puts the amount of electricity the vehicle consumes into perspective. If you’re trying to charge an EV at night using stored solar energy, you are essentially transferring electricity from one battery into another battery. And that can drain your home battery very quickly.
Should You Add More Home Battery Storage for an EV?
It depends primarily on when you charge. If you’re able to charge primarily during solar production hours, you may not need substantially more home battery capacity. The solar panels can help power the vehicle directly. If you are going to charge almost exclusively at night, additional battery storage may make sense. Let’s say your house normally consumes approximately 20 kWh overnight. Then you add an EV that needs another 30 kWh. Your total nighttime electricity requirement could now approach: 50 kWh. A battery system that was perfectly sized for the house before the EV may now be significantly undersized. This is why we look at solar and battery sizing together. It is not enough to simply add more panels. We have to determine where that additional solar production is going and when it will be used.
You Usually Shouldn’t Size Your Home Battery to Fully Charge the EV
There is another important distinction. If your EV has a 75-kWh battery, that does not mean you need 75 kWh of additional home battery storage. That usually would not make financial sense.
Instead, we want to look at your actual habits.
- Maybe you only need to replace 20 kWh on a normal night.
- Maybe you charge 30 kWh every other day.
- Maybe you can charge at work twice per week.
- Maybe you’re home on Fridays and can charge directly from solar during the day.
- Maybe you only drive 30 miles per day and don’t need to charge every night.
We want to size around your normal consumption habits, not the theoretical maximum capacity of the EV battery.
Daytime EV Charging Can Make a Big Difference
If you have the flexibility to charge during the day, that can change the entire battery calculation.
Instead of doing this: Solar → Home Battery → EV, you may be able to do more of this: Solar → EV.
That reduces how much energy needs to be stored before reaching the vehicle. Modern EVs and chargers also have scheduling capabilities, which can allow homeowners to control when the vehicle starts charging. This can be extremely valuable when coordinating the EV with solar production and utility time of use rates.
Level 1 vs. Level 2 EV Charging
Another common question homeowners have is whether the charger itself changes the amount of solar they need. There are 2 common residential charging options.
Level 1 Charging
Level 1 charging typically uses a standard 120-volt outlet. It is simple and does not require a large, dedicated charging circuit, but it is relatively slow. For someone who does not drive very much, Level 1 charging may be enough. For homeowners who drive regularly, it may not replace the vehicle’s energy quickly enough.
Level 2 Charging
Level 2 charging typically operates at 240 volts on a dedicated electrical circuit. Think about some of the larger loads in your home. Your air conditioning system, electric dryer, electric oven, pool equipment, and other larger appliances typically have dedicated circuits. An EV charger is similar. A Level 2 charger allows substantially faster charging and is generally the more common option for homeowners who regularly charge an EV at home. But there is an important distinction. Charging faster does not necessarily mean you are using more electricity overall. If your vehicle needs 30 kWh, it needs approximately 30 kWh whether you put that energy into the vehicle quickly or slowly. The difference is how much power the charger is pulling at one time. That becomes important when we look at the home’s electrical panel, battery output, inverter capacity, and overall electrical design.
Your Electrical Panel Needs to Be Evaluated Too
When we add an EV charger to a home, we also need to evaluate the electrical service.
Some of the things we look at include main panel size, main breaker size, available breaker space, existing electrical loads, EV charger amperage, and whether some form of power management is necessary. Some homes can easily accommodate an EV charger. Others may require electrical upgrades or load management equipment. This is another reason it makes sense to tell your solar contractor about the EV before the system is designed. We can evaluate the solar, battery, EV charger, and electrical panel together instead of treating them as separate projects.
What If I Already Have Solar and Then Buy an EV?
This happens all the time. A homeowner installed solar 5 years ago. At the time, the solar system was sized correctly for the home’s electricity consumption. Then the homeowner buys an electric vehicle. A year later, the homeowner notices the electricity bill has gone back up. They may assume something is wrong with the solar system. But when we look at the numbers, the solar system is producing exactly what it was designed to produce.
The problem is that the home’s electricity consumption has increased. If you had a system designed to produce 10,000 kWh annually and then add an EV that consumes another 4,000 kWh, the original solar system was never designed to cover that additional electricity. In that situation, additional solar may be required. Depending on the homeowner’s existing net metering agreement, we may also look at adding battery storage and designing a non-export solar addition so we can address the increased consumption while protecting the existing net metering agreement.
What If I’m Planning Solar and an EV at the Same Time?
This is actually the ideal scenario. We can design everything correctly from the beginning.
If you’re planning on purchasing an EV, your solar installer should know:
- How many miles you drive each year.
- Which vehicle you are considering.
- How much of your charging will happen at home.
- Whether you can charge during the day.
- Whether most charging will happen at night.
- Whether you have charging available at work.
- Whether you plan on purchasing another EV in the future.
Those questions allow us to build a much more accurate electricity model.
Let’s Look at a Complete Example
Let’s say your home currently consumes: 10,000 kWh per year
You purchase an electric vehicle and expect it to add: 4,000 kWh per year
Your expected annual consumption is now: 14,000 kWh
If we want to design around approximately 25% additional production: 14,000 × 125% = 17,500 kWh
Now we need to design a solar system capable of producing approximately 17,500 kWh annually based on the actual production characteristics of the property. Then we look at the battery. If you charge your EV primarily during the day, the additional battery requirement may be relatively small because the vehicle can consume solar production directly. If you charge primarily after 6 PM, the calculation changes.
Now we have to determine how much electricity the home uses overnight, how much the EV will consume, and how much battery capacity is needed to minimize grid usage. That is the complete way to look at the system. You cannot size the solar panels without considering consumption. And you cannot properly size the battery without considering when that consumption occurs.
Is It Better to Charge an EV With Solar?
In many cases, it is absolutely better to charge an EV with solar energy. This is one of the biggest advantages of combining an electric vehicle with solar. Instead of purchasing gasoline every week, you’re using electricity to power your vehicle. And instead of purchasing all of that electricity from the utility company, you now have the ability to generate a large portion of it at your own home. You’re essentially using sunlight to help power your transportation. But the economics work much better when the system is designed correctly. If you install solar based only on your current electricity bill and then purchase an EV 6 months later, the system could immediately become undersized. That is why future planning is such an important part of solar design.
So How Much More Solar Do You Actually Need for an Electric Car?
For many homeowners, planning for approximately 3,000 to 4,000 additional kWh per year for an EV is a reasonable starting point. For higher mileage drivers who charge almost exclusively at home, that number could be closer to 5,000 to 6,000 kWh per year or more. Depending on the solar production available at your property, that could translate into approximately 2 to 4 additional kW of solar. With modern panels around 450 watts, that could mean approximately 5 to 9 additional solar panels.
But panel count is only part of the calculation. The bigger questions are:
How much do you drive?
- How much do you charge at home?
- How much electricity does your vehicle consume?
- When do you charge?
- And how much of that charging do you want your solar and battery system to cover?
At Supreme Solar and Electric, these are the questions we look at when designing solar for a homeowner with an electric vehicle. We look at the home’s existing annual consumption, future consumption, EV charging habits, roof production, electrical panel, solar system size, and battery capacity. Because the goal should not simply be to install more solar panels.
The goal is to design the entire energy system correctly, so the solar panels, home battery, electric vehicle, and house all work together. If you already have an EV or know you’re going to purchase one in the future, account for it before installing solar. It is much easier to build the additional electricity into the system from the beginning than to find out later that your new EV caused your solar system to become undersized.