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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