Short answer: not quite this much

Driving an electric vehicle (EV) is one of the most enjoyable ways to do right by Mother Earth. And charging that EV with power straight from the giant fusion reactor in the sky makes it doubly as fun. The question is, how much solar do you need to charge an EV? Another important question is: what does it cost to charge an EV with solar? Let’s find out together!

Note: we’re not talking about cars with solar panels in their roofs, here. This is an article about charging the market’s top electric cars using solar panels on your home’s rooftop.

Take a look at this infographic (and if you want to learn more, we dig way down deep in the article below):

Gas vs. Solar EV charging infographic

How do all those numbers work?

This all depends on a few factors: the kind of electric vehicle (EV) you have, its battery capacity, and how much sun you get where you live. See, no EV is exactly like another, and solar production varies widely depending on where you are in the country. Let’s dig in and see what we come up with!

The car

What we call an EV is important to this calculation. We love plug-in hybrids as much as the next environmentally-minded guys, but let’s look at only those cars that can go at least 50 miles in pure electric mode. That’s enough to get most people to work and back every day, and cutting it off there helps make the list manageable—cutting out plug-in hybrids like the Prius and Honda Accord that can go just a couple dozen miles in EV mode.

After a cursory glance at an excellent car website called “Plugin Cars,” we came up with a list of 14 current and very near-future EVs that meet our criteria. Here they are:

  • BMW i3
  • Chevy Bolt
  • Chevy Spark EV
  • Fiat 500e
  • Ford Focus Electric
  • Hyundai Ioniq EV
  • Kia Soul EV
  • Mitsubishi i-MiEV
  • Nissan Leaf
  • Smart Electric
  • Tesla Model 3
  • Tesla Model S
  • Tesla Model X
  • Volkswagen E-Golf

14 electric vehicles in 2017

Gee, the auto industry likes to pose its cars the same way for promo shots…

Pretty sweet list, huh? Now the things we care about when it comes to those cars is range (at least 50 miles electric, remember?) and battery size. We’re guessing you’re gonna be charging up from empty every day on some of the EVs with smaller batteries. If you’re driving your Tesla Model S 200 miles a day (which in L.A. County seems totally doable), it’s gonna take a lot more power than charging the Volt you drove from Bloomington, Minnesota to Minneapolis and back.

But comparing a 200-mile charge with a 50-mile charge doesn’t really tell you the right story. By calculating the ratio between each car’s battery size and range, you come up with a figure we’ll call Efficiency. That’s shorthand for energy required (kWh) to go one mile. Multiply that by a standard number of miles per day (let’s call it 50 to make it easy, and you’ve got Daily Charge (kWh/50mi). Simple!

Check out how the field ranks below (lower numbers are better. The Chevy Spark wins!

How much electricity it takes to charge an EV to drive 50 miles per day.

 

 

Click to see the numbers that went into this image

 

As you can see in the chart, the daily charge champion is the Chevrolet Spark, which needs just 11.5 kWh of juice to go 50 miles. The Tesla Model X tips the scales on the other side of the field, needing 18 kWh for its daily commute. The average daily charge for these electric vehicles is 15 kWh, which, multiplied by the nation’s average energy price of $.13/kWh means driving 50 miles will cost you just $1.95. Take THAT, gas prices! (But solar is even cheaper—read on to discover how much!)

So there you have it; now you know how much energy you’ll need for these guys every day. Of course those numbers go up or down based on usage, so to calculate your own needs, divide the number for your car by 50 and multiply by how many miles you drive every day.

Our next step is to figure how much solar you’ll need to make the electricity for your car.

The solar equipment

Yeah, I’d live there.

Right up front you have a choice to make. Do you want to be pragmatic and decide that a kWh of solar is a kWh of solar, whether it’s sent to the grid or used to charge your car’s battery? Or do you want to make absolutely sure every kWh used for the car comes from solar, even though it will cost more money?

The decision here is between a solar panel system on your roof, or solar plus storage: batteries that capture the kilowatt-hours and can be used to charge your ride when the sun isn’t shining. We pragmatists know that a kWh generated is a kWh earned, and we’re happy to send our panels’ generation off onto the grid and draw power from the utility company at night to charge our panels.

But calculating the cost of solar without battery storage and trying to figure out what percentage of that goes into charging your car if you plug it in at 5 pm or 7 pm is way too hard. We’re pragmatists, remember? So we’re going to assume since you have an EV and you want to charge it with solar, you’re gonna get some batteries to do it. Easy peasy. Sort of.

Electricity needs and the cost of solar+battery

Different parts of the country get different amounts of sun (duh), so you’ll need more panels to fill up your EV’s battery if you’re in Poughkeepsie than you will if you’re in Phoenix. Estimates of potential solar energy generation are based on how much sunshine reaches the rooftops where you are. Scientists and other cool people call this concept “insolation.”

Luckily, our country’s very smart scientists are experts at measuring insolation, and they even do it in a very handy way. This map of the US produced by the National Renewable Energy Laboratory shows insolation measured as kWh per kW of solar panels per year:

Darker means sunnier, dude.

All you do is find the number of the shaded area covering your home’s location, multiply it by .78 (for energy losses from wiring and AC/DC inversion) and multiply that number by the number of kW your solar system is rated to produce. Conversely, if you want to figure out how big a solar installation needs to be to produce your Tesla’s 18-kWh daily needs, you divide that number by the product of the area and .78, then divide the result by 365 days to get the final number.

Is your head spinning yet? Let’s work through an example:

You live in San Diego and you drive a Nissan Leaf 50 miles a day. How much energy do you need to charge it daily?

  • Your Nissan Leaf needs 14 kWh/day
  • A 1-kW solar installation in your city produces 1,482 kWh per year (get that number by taking 1900—the area of the map above where San Diego is, multiplied by the .78 for electricity losses)
  • That’s 4.06 kWh/day (1,482/365 days)
  • You need a 3.44-kW solar installation to feed your Leaf (14 kWh per day/4.06 kWh produced per kW of solar)

Now let’s break that down in a quick little table that shows how much solar you’d need to charge the average EV enough to go 50 miles (remember that’s 15 kWh, from the above table), depending on where you live in the US:

Size of solar installation needed to produce 15 kWh per day:
  Pacific Northwest Northeast Midwest South/Southeast Southwest Average
System Size 7.2 kW 5.2 kW 4.7 kW 4.4 kW 4.3 kW 5.2 kW
# of 250W panels 29 21 19 18 18 21

Size of solar installation needed to produce 15 kWh per day:

Now we get to find out much each kWh will cost ya. Don’t look so excited.

 

 

The cost of solar

Let’s go with some simple assumptions for the cost of panels, inverter, batteries, and more. The nationwide average for the cost of solar equipment is $4/watt. You’ll also need two Tesla Powerwall 7-kWh batteries for charging the car, which cost $3,000 each plus $1,000 for installation. So we’ll multiply each system size by $4,000 and add $7,000.

But then there’s incentives, which vary all over the country. We’re going to make it easy on ourselves and assume at least the federal solar tax credit, which will knock 30% off the cost within the first year or three depending on how big a tax burden you have. So we’ll take the size from the last step, multiply it by $4 per watt, then take away 30% of the results for a cost after incentives.