Design & Relationship Between Electric Cars & Solar



To recap just a bit, we’ve arrived at zero net cost for the energy required to live in our home and drive our two electric BMW i3’s powered by Solar PV. Our Solar PV system, installed in 2007, and our EV driving beginning in 2009, led to our system being fully paid off in April of 2012. For our family, that’s an ongoing savings of around $8000 a year for the next few decades at least.

But there’s another side of the story to tell, a story of design scale and the relationship between the car and the source of fuel that’s both surprising and fascinating.

*Editor’s note: This post also appears on Peder’s blog. Check it out here.


Good design brings visual beauty to our cities, buildings, and spaces as well as to the automobile. There is a natural scale, for example the scale of an object as compared to a person, such as the height of a ceiling. Or the proximity of an object to another object or person, such as landscaping, that when done well provides a sense of comfortableness, a feeling of beauty. It works well and fits together nicely.

When not done well, a poor design scale can seem awkward, weird, and uninviting. “What the heck were they trying do here” is a typical response resulting from poor design scale.

Now let’s take a look at the design scale between the car, a parking space, and the space required to power the car a typical 12,000 miles a year.

An average parking space for a car is 9 ft. by 18 ft. for a total of 162 ft. An average residential garage space per car is even greater at more than 200 sq. ft.


From our real world experience, we know that our car the BMW i3, requires 2860 kWh to drive 12,000 miles. We also know that for each kw in system size our Solar PV system located in coastal California produced 1588 kWH. For our individual situation, a Solar PV system size of 1.8 kw is required to provide the electricity to drive our BMW i3 12,000 miles a year.

For a better comparison, let’s design an average system size that will work for the majority of electric cars in all areas of the USA.

The efficiency of electric cars range from 3 miles per kwh to 4.2 miles per kwh. A Solar PV system size of 2.34 kw would cover all of these different choices in cars.


Additionally, our solar resources in our coastal California location is 5.25 kWh per sq. meter, per day. The lowest in the nation is around 4.00 kWh per sq. meter, per day. Factoring in the lower number, takes the above 2.34kw solar PV system size to 3.06kw.

To round off nicely, a 3kw system size will power the vast majority of eclectic car choices 12,000 miles a year, anywhere in the USA.

How large is a 3kw system? The SunPower X21 panel produces 345 watts and is 17.57 sq. feet in size at 61.4inches by 41.2 inches. 8.7 panels are needed for a 3kw system size but let’s call it 9 panels for 3.1kw system to keep the math easy for a total of 158 sq. ft.

  • A typical parking space for most cars anywhere in the USA is 162 sq. ft.
  • A Solar PV system in coastal California that powers a BMW i3 is 92 sq. ft.
  • A Solar PV system that powers most cars anywhere in the USA is 158 sq. ft.

That’s a beautiful proportional design scale. The size of the space required to park a car is larger than the space it takes to provide power for the car.

Going a bit further, in most cities, there are 4-5 parking spaces per car. One at home, one on the curb, one at work, and one or two out in the city for your use as you shop and run errands. All we need to do is pick one of those 4-5 parking spaces and put a Solar PV system on top and you’re good for 12,000 miles of driving.


We can power our cars in roughly 25% of space that is required to park our cars throughout a city. We can do so with Solar PV at a fixed cost equivalent of $0.50 – $0.75 per gallon of gasoline.


A relationship can be a coupling, cause and effect, harmony between two or more in the way they are connected, a symbiotic reliance on the other.

Any car needs an energy relationship to move.

Currently most cars have a required relationship with a gas station owned by someone else. You simply must fuel your car at a gas station. The gas station stores and dispenses the fuel that arrives by tanker truck from the refinery. The refinery receives the crude oil from a tanker or oil pipeline. The oil tanker receives the crude oil from a pipeline or tanker truck that receives it from an oil well or oil source.

This conveyance chain of hydrocarbons to your car can be thousands of miles long traversing many oceans. In this carbon chain you are the payer at the end of the chain with all the owners of the pieces of the chain enriched by your payment.

This long hydrocarbon conveyance chain is also very inefficient, resulting in less than 20% of the original energy contained in the crude oil reaching the wheels of your car where they meet the pavement.

The electric car also has a required relationship with electricity. This can be made from many more sources including Solar PV that sits on the roof or carport above one of the parking spaces provided for the car. The conveyance chain can be as short as 30 feet, from where the sun shines on the Solar panel to your car.

This electricity harvested from the sunshine supply chain to your car is owned entirely by you and enriches you. This conveyance chain supplies the electricity to your car where your car is parked and results in over 85% of the original sourced electricity reaching the wheels of your car where they meet the pavement.

That’s a beautiful relationship between the car, electricity and nature that ultimately enriches you the owner. It’s nice to know that not one square inch of new land is needed to power these cars with sunshine. We can make the electricity very simply and efficiently at any of the multiple locations in which the car is parked.


Relationships and design…They matter.

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38 Comments on "Design & Relationship Between Electric Cars & Solar"

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Does it really cost $666/month ($8000/12 months) for electricity in California? That’s incredible. At rates like those, it’s a wonder that every home in California isn’t solar already!

Nicely written blog post.

I think he’s adding in gas savings there as well.

The average KWH for standalone single family home is 533KWH/mo. In PG&E territory the rates is an average of $appx. $0.18/KWH although that is rising by 7% per year and will rise by 46% in 2016 due to recent rate changes the utilities have been pushing for.

Are you saying that electricity costs in CA are increasing faster than gasoline?

Holly smokes, time to buy solar panels before they run out or drain the sun 😉

actually higher utility charges for energy make onsite (self) generated energy MORE interesting as it saves you more.

It’s kWh, not KWH. 🙂

KWH sounds gangsta 😉

Pg&e in CA has tier pricing so while some of it may start at 18c most of the electricity you pay more like 30c / kWh unless you consume way below average. Still not likely to be $660/month but easily above $200 in the summer.

SCE charges 5 cents per kWh to transmit and 10 cents to produce when below the baseline.

Another great reason to have work parking lots covered with PV.
Or better, work at home saving 50% of car travel energy.
Now I know it is hard to believe but gas cars only get 7% of their fuel’s energy to move the car as it never runs in it’s eff state.
For instant an EV can drive 50-60mph on the energy needed for a gas engine to idle, going nowhere.

Can you share the details please about “For instant an EV can drive 50-60mph on the energy needed for a gas engine to idle, going nowhere”?

The engine friction, water pump, alternator, cam, valvetrain, oil pump, belts, etc takes a lot of power just to turn over.
It’s 5-15hp depending on engine.
Vs an EV motor that just has 2 roller bearings and only uses, makes power when needed.

I thought the Prius was designed so that its 40% efficient motor runs at it’s most efficient state as much as possible.

The Prius tries but a gas motor is so far from power needs of a vehicle it still isn’t that good.
The problem is the motor rarely runs at more than 30% of it’s capacity and still has the engine friction, just less of it.
Vs an E motor starts at max torque only using the energy needed to move the car, not a lot of parasitic energy of a gas motor.

Let’s also not forget that the energy required to refine a gallon of gasoline is enough for a typical EV to travel something like 20-30 miles.

That is one of the key factors often overlooked. The electrical cost of refining.

Without a storage solution, residential solar for EVs is not sustainable. Net metering has limited capacity, and few people charge mid-day. However, solar powered workplace charging makes a lot of sense–employees can charge mid day without any storage needed.

Both are true.

Workplace charging needs to find a way to adopt solar covered canopies just as Peder suggests.

Long term, residential will need battery storage, but not as some think. The ideal solution is just enough to eradicate peak load. That means enough battery to extend 4-5 hours to around 8-9:00 PM.

I don’t even care if the utility takes control of my battery (Bill H will jump on me for this.) Utilities spin about the cost of transmission, when this is peanuts compared to the cost of designing to handle peak load.

So yes, residential solar WILL be sustainable because they WILL adopt the storage battery size aimed at peak load. Tesla Motors is aiming directly at this equation with a 7kWh Powerwall pack.

Yup net energy metering is doomed. Nevada is out. Arizona has steep fees. Big utilities in CA are heavily opposed. This entire article assumes NEM is forever.

No, the key point of the article is that there should not be a problem powering lots of electric cars, because the total parking space they occupy has greater PV potential than the total energy they consume.

A bit of a simplification, but the principle is correct.

It’s important because if electrification is successful it would imply cheap batteries, which would be important in any shift to renewables.

I agree that the NY State restriction of 110% of your previous usage is a bit of a pain, but I put on the application that I just bought 2 – high – usage EV’s, and that my electric consumption was going to skyrocket, therefore I Could install a 9120 watt system.

I’m way overbuilt – I make around 9300 kwh per year due to the lack of sunshine, but it has the advantage that the utilities don’t fight solar as much as they do down South because Solar Power is no where near as compelling ( or threatening to the utilities ) as Arizona or Florida. But this year I have much larger Christmas lighting usage than in previous years, so I shouldn’t have anywhere near as much of a credit this year than last. I dislike having too much of an annual credit since they might come back and say I was exaggerating on my application, so this year I’ll try to use most of it up. I now have 2 very efficient EV’s though, so I’ll have to use the car heat as much as possible to increase their charging times.

“Without a storage solution, residential solar for EVs is not sustainable.”

Given the grid, and neglecting transmission losses, does it really matter whether the solar panels are at home or at the work place? Isn’t it enough that you can plug your car into the grid at work while you panels are sending electricity into the grid at home?

To some degree yes Lindsay, but it still does not deal with the peak load hours that continue after the sun goes down. To be part of the complete solution, the EV-PV users need only store 4-5 hours worth of solar energy to do their part in minimizing peak load demands. Right now, it is not a problem due to the small percentage of solar adopters, so yes I agree with you 100% that clean energy provided to the grid is the same. There are some areas in Hawaii, California and Germany that will reach a solar saturation levels sooner than others. The reason I am leaning toward adding a Powerwall size battery in the future, is that I know it nullifies all reasonable arguments by the utilities about me not covering my fair share of cost. Look any utility guy in the eye, and ask what percentage peak load support adds to the cost of generation. And unless you don’t have any neighbors closer to you than you are to the utility, you are also minimizing transmission losses as your excess power goes to you closest neighbor. This article, and the last written by Peder Norby,(see staff link) has… Read more »

My 5.1kW DC panel array works quite well at giving me enough net electricity to power my house and my Volt year-round.

And as noted, it’s beneficial to the me AND the utility. I charge my EV at night, when there is ample energy available on the grid, and I produce excess power during the day, when there is higher demand, which benefits the utility.

Aside from the shoulder of the peak after the sun goes down, this works out very well. And that shoulder only becomes an issue once some critical PV installation amount is reached (still a ways off).

Great write-up!

we have a solar system installed on our house. fed and state tax credit along with utility co incentive was based on our 1 yr previous elec usage. MORONIC. now since i purchased an ev the output doesnt even come close to meeting my updated elec needs. the only way to increase my output is with add pv panels which dont qualify for any incentives. the inverter also has to be changed for add. capacity. EXTREMLY COSTLY !!! TRULY MORINIC. With gas at record low prices ev’s make less sense than ever, based on nys high elec rates

you should be able to get credit for the expansion.

If gas stayed at record low $2/gallon for 25 years, it would cost $61K to drive the average 2 cars at average 13K miles per year averaging 32 MPG for both with a 3% annual increase.

A 4kWh solar array would cost roughly $11K after incentives to handle the energy needed for 2 EVs over the same 25 years.

So your record cheap $2 gas is going to cost you $50K more over the same period. BTW, it’s not going to remain $2.

This is an intellectual site ampzilla, so you have to do better than that…..

To help increase the intellectual level a bit more I would appreciate it if you wrote lower case “k” = kilo = thousand instead of “K” = Kelvin.

Unless it’s cold cash we’re talking about then I guess some temperature reference would be fine. 🙂

Yes of course the power rating of the array, not the energy. My apologies. The horror to wake up and read what you post in the festive hours of the holidays. Was at a party listening to why $2 gas will stop EV sales, then opened this excellent article by Peder Norby, only to read the same snark that I was listening to. Don’t drink n drive, and don’t party n post….

SolarCity lost our business because they said they could only install 125% of previous years capacity..eventually found an installer that wanted our business..they had no problem having us sign a waiver for a “jumbo” system..

Seems like you should install another solar electric system beside the first one.

Nicely done Peder! Your PV + EV insights are always a very interesting read!

Thattttt was a really great article, really makes you think.

I hate to say, my employer is ahead of me. Our parking lots are well covered by panels. I go by the power center that collects from those, and look at what it is generating on a given day.

It always struck me what a perfect solution it is to have a car port covered with solar panels. A covered parking spot at work or home is ideal, since it slows the aging of your paint, and keeps you out of the rain.

Conversely, my city makes it illegal to have covered car ports, since the city wants no structures beyond the front of the house.

Call it a porch or patio instead of a carport or use solar as window awnings.

Excellent article and real life example Peder. You can convince most people with your great logic.

My wife and I drive nice and easy and get 4-6 miles or more per kWh. Our 4 Kw system runs our house and electric vehicles with 10% extra we sell back. We use no other fuels on any types except Solar. YOUR QUOTE=The efficiency of electric cars range from 3 miles per kWh to 4.2 miles per kWh. A Solar PV system size of 2.34 kw would cover all of these different choices in cars.


Thanks once again for an in-depth article full of useful real world numbers!

However, I do have one issue with your analysis, where you say:

“To round off nicely, a 3kw system size will power the vast majority of [electric] car choices 12,000 miles a year, anywhere in the USA.”

But that’s using a figure for the average daily insolation (that is, the amount of sunshine an unshaded roof will receive). Unfortunately, in most areas, it gets cloudy on some days. According to the rule of thumb I’ve seen on various “off-grid living” websites, you need four times as much surface area in your solar panels to generate the same amount of energy on an overcast day.

If you’re using net metering, and averaging out for each month the amount of energy your solar panels generate vs. how much it takes to charge your EV, then yes, the statement is true. But if you want to be able to actually power your EV entirely from rooftop solar panels, then you’ll need about four times as large an installation. Of course, if you have a battery backup system, that will reduce the amount of extra area needed.