How Many Solar Panels Do You Need to Charge Your Electric Car?

An EV is the single biggest reason a household's solar math changes. It can add 3,000 to 5,000 kWh of demand to your home in a single year, which is more than what a lot of houses use for everything else combined. If you plan solar without the car in mind, you will undersize. If you plan for the car but not for where you live, you will still undersize. This article is the math for getting it right.

How much electricity does your EV actually use?

Start with the one number that matters: how much electricity your car eats per year. That depends on two things, how far you drive and how efficiently the car runs.

The average American drives about 12,200 miles a year. At that level, most EVs need roughly 3,000 to 5,000 kWh of electricity annually. The efficiency spread is real. A well-tuned sedan class EV can sip around 27 kWh per 100 miles, while a heavy truck or SUV class EV can push 40 kWh per 100 miles or more. So a compact EV doing 12,000 miles lands near 3,300 kWh, while a big electric truck on the same miles can top 5,000 kWh.

A quick rule of thumb for your own car: multiply your yearly miles by your car's kWh per 100 miles, then divide by 100. If you do 14,000 miles in a car that uses 30 kWh per 100 miles, that is 14,000 x 30 / 100, or about 4,200 kWh a year.

The one formula to size it

Once you know your EV's yearly kWh, the sizing math is the same formula used to size any solar system:

System size in kW = yearly EV kWh / (peak sun hours x 0.86)

The 0.86 is the real world derate for inverter losses, heat, soiling, and wiring. Peak sun hours is the number that is specific to where you live, and it is the part most people get wrong.

Where you live matters more than the car

Peak sun hours swing hard by region. Rough numbers:

• Phoenix, AZ: 6.0 to 6.5
• Las Vegas, NV: 6.0 to 6.5
• Austin, TX: 5.0 to 5.5
• Denver, CO: 4.5 to 5.0
• Dallas, TX: 4.5 to 5.0
• Atlanta, GA: 4.0 to 4.5
• New York, NY: 3.8 to 4.3
• Portland, OR: 3.2 to 3.8

Now run the same 4,200 kWh EV through the formula in three cities:

| Location | Peak sun hrs | System needed | Rough panel count (400W) | |---|---|---|---| | Phoenix | 6.2 | 0.8 kW | 2 panels | | Austin | 5.0 | 1.0 kW | 3 panels | | Portland | 3.5 | 1.4 kW | 4 panels |

That is the whole point of this article. The same car, the same mileage, needs almost twice as many panels in the Pacific Northwest as it does in Arizona. If you plan your solar around a car bought in one climate and install it in another, you will be guessing.

You probably need more than just the car

Here is the trap. The formula above sizes solar for the EV alone. But you also have a house running lights, appliances, a fridge, maybe an AC. If you add the car on top of your existing usage, most EV owners end up targeting a larger array than the non EV home next door.

A practical target: size for your current home usage plus the car, not for the car in isolation. A household that uses 4,000 kWh a year and adds a 4,200 kWh EV is now looking at roughly 8,200 kWh. In Austin that is about 2 kW for the house and 1 kW for the car, so a 6 to 7 kW system is a reasonable target. In Portland, push it toward 8 to 9 kW.

The timing problem, and the cheap fix

Solar makes power during the day. Most people charge their EV overnight. Left alone, your solar car charging is really grid car charging. You have three fixes, in order of cost:

1. Charge during the day. If the car is home in daylight, just schedule it for midday. Free and simplest.
2. Add a solar aware charger. A charger that watches your solar surplus and only draws into the car when the panels are making extra power. These run roughly 600 to 1,500 installed.
3. Add a home battery. Store the day sun and charge at night. Most flexible, but it is the expensive option, and for most people it is not required to get the benefit.

For a lot of homes, a solar aware charger plus a cheap overnight EV rate covers the whole problem without buying a battery.

Why the savings are bigger than they look

Charging at home on solar can cut your per mile fuel cost by 50 to 100 percent versus gas or public charging. EnergySage's comparison for a typical sedan doing 12,200 miles a year: home charging on solar is essentially free for the fuel, home charging without solar is about 589 a year, public charging is about 1,234, and a gas car is about 1,365. Over 25 years, that gap is in the tens of thousands of dollars.

The second benefit is the one that makes solar more valuable, not just cheaper. An EV is a large, flexible daytime load. It soaks up solar you would otherwise be exporting for pennies. Adding the car raises your self consumption, which is the single biggest driver of whether your solar pays off, especially in states that pay you less for exported power.

What to do before you buy the panels

Two things to nail down first.

First, if you are even considering an EV in the next year or two, size for it now. It is cheaper and easier to install enough capacity upfront than to expand a system later.

Second, stop estimating peak sun hours from a generic chart. The number that decides how many panels you need is the one for your exact address, your roof orientation, and your local climate. That is the difference between a system that covers your car and one that barely dents the bill.

You can get your home's actual sunshine hours, a recommended panel count and wattage, and an estimated system size and cost for your specific address at https://solrscan.com for 19 dollars. Plug in the address, see the number for your roof, and then add your car's yearly kWh on top. That is how you size a system that is big enough for the life you are actually planning, not the life you had last year.