Wednesday, August 14, 2013

The BMW i3 and a Solar PV Roof ?

As both a BMW customer and a SunPower customer, with a few free hours for ideation (fancy word for daydreaming,) I paired these great companies and products together and it energized my eager mind.

Is it time to start looking at a solar roof for the i3?   

The BMW i3 is lighter and more efficient than any other 4 passenger electric car in production, thus it can drive more miles per kwh (somewhere around 5 mpk.) Solar PV is ever increasing in it’s ability to crank out more electricity for less cost in less space. The SunPower X21 panel is the world’s most efficient consumer panel rated at 345 watts and is 61” by 41” in size.   

61” by 41” strikes me to be very similar  in size as the general dimensions of the roof on the BMW i3. Is it time to start looking at a solar roof for the i3 and for BMW to talk to SunPower?

SunPower X21 cells on Stanford's solar racer.
Let’s break it down and find out!

A SunPower 345 watt panel will produce 530 kwh per year at an optimum angle and orientation in California  (1kw = 1550kwh production per year.)  If you assumed the BMW i3 would have the same exposure to the sun as your house roof, you would most likely de-rate about 15% due to the flat pitch of the BMW i3 roof leaving you with approximately 450 KWh per year.   That amount of energy will power the BMW i3 for 2250 miles a year.  (450kwh times 5 miles per kwh.)   That would be around 20% of the total energy required for your typical 12,000 mile a year driver.  

A realistic scenario for a traditional 9-5 worker with a constant workplace  is that your car is driving in the morning and afternoon when the solar irradiation is very low and parked at work for most of the daylight hours.   Assuming a nice non shaded parking space, a de-rate of 33% would be about right, leaving you with 354kwh of generation annually, good for 1770 miles of driving a year.  

An even more realistic scenario that is blended, is that some days including the weekends your car is in the garage all day, some days you are in the shade but most days in the sun, and some days you’re on the road several trips a day with all sort of shading variations.  In this scenario a de-rate of 50%  or 275kwh of generation annually, good for 1375 miles of driving a year.

In my scenario,  I do park outside and there is lots of sun, I would estimate around 300kwh a year or 1500 miles a year could be powered directly by  a solar powered roof rated at 345 watts.

The SunPower panel is running about $2.00 a watt thus a retail panel cost of around $700.    I’m sure there are significant cost in affixing individual cells to the CFRP roof and mono crystalline cells are not flexible but I would imagine a custom mold where the cells could be connected and sealed and then that outer shell affixed to the roof of the car.  An Inverter of some sort would be required and some crazy ass smart engineering and software controls added. (Because I want to see how much electricity my cars roof is generating on my i-drive.)  All in all,  I see no reason why this option should cost more than $3000 for a high production vehicle.

Would I pay $3000 for this option?  Heck yes!  1000-1500 mile a year free for the 10 year life of the car.

Does it save me money over roof top solar on the house? Heck no.  That $3000 panel on the car is generating half on the electricity of the $700 panel on your roof.  

Is flexible thin film a better solution? Perhaps because cost would be greatly lowered but electricity production would be only 60% of the x21 cells.

I say bring it on as an option BMW!  Then again, it would be a shame to hide all that beautifully exposed carbon fiber on the roof of the car and not be able to have an operable sunroof.

The dawn of a true solar 
powered car is nearing.  At least in part.



  1. I agree it would sure look cool, but as you explained, the economic feasibility will probably delay it for a while. Powering the Fit EV to go 1500 miles costs only $50 at current electricity rates, but since installing solar panels, it's considerably less. Maybe for the i8 customer?

  2. Really nice blog!! I am glad to read your post, pretty informative..thanks for providing such a nice post.
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  3. We are BMW M3 lovers! We teach you how to wash them, clean them, fix them, drive them, and we have every used Mirrors part you will ever need for your M3! – Like AreliU

  4. This is such a great and informative post.You have done a great effort here by posting the BMW here.I love your effort so much .
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  5. Food for thought: cars operate on a battery, batteries run on DC, alternators run on DC (if you applied logic to a hybrid), but most importantly PV runs on DC as well. Now my question is why must there be an inverter? Hook that bad boy straight to the battery and throw a couple meters somewhere on the line to measure productivity.
    Second thought: that $3000 option for the car earning "1000-1500 mile a year 'free' for the 10 year life of the car," has a major flaw. Besides that fact you said free and $3000 in the same sentence confuses me but also that totals out with $2 a mile at best, I'd take a taxi. With a gas burning car with 40 mpg at $4 a gallon that same 1500 miles would cost $150 dollars, 10 cents a mile. You would be paying 20 times the cost of gas and that's before you ever think about replacing those batteries. I truly do love the idea of PV cars, boats, houses, planes... well everything, but unfortunatly the idea of a PV car will have a greater impact on the world (cause batteries really are not friendly), a greater impact on the budget (because PV doesn't work well in the shade) and could be time better invested elsewhere. Try a bicycle, no emissions, no dead batteries and great for the blood pressure.

  6. Thank you so much for posting. I have been looking for something like energy

  7. I bought the i3 May 28th (Hudson Valley BMW) - the first one to be bought off a show room floor in the US. I love the vehicle! I can milk 100 miles out of it on a single charge for some destinations. The roof actually looks like it has small solar cells embedded within it! I wish BMW would put cells in the roof and at least power the accessories with it, perhaps the air and heat as well. As it is, I often drive with the windows cracked in the summer and in the winter I plan to use the seat heater. I also keep the car in the garage which helps a great deal. I have a little fold up solar panel laying on the dash that charges a battery which in turn charged my iPhone with no problem. Fill that roof with mini-x21 cells and you should be able to power the air and heat on sunny days. I am in the process of putting 64, yes 64, E20 SunPower panels on my house, which will completely charge the car and run the house. But to have a roof filled with built in SunPower cells would be a great way to gain mileage and be comfortable at the same time. I often drive with the windows cracked in the summer - and rarely ever go out of Eco Pro+ mode (thus I do not use any air conditioning). It is not that bad, considering I keep the car in my garage.

  8. the ford cmax energy with the solar roof they found heated up the interiorof the car far too much to be an option.

  9. Far better to make some solar topped shade for parking lots. I'd rather pay $0.70 a watt on a parking lot shade than $7 a watt on the roof of my car where I would feel guilty every time I looked for nice shaded parking.

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  11. Hello. I did a search that brought me to this old posting. I am looking into purchasing an i3 and was wondering if a flexible panel could be affixed to the roof and if it might be worthwhile. After reading this thread I wonder how much flexible panel cost to efficiency might have improved since 2013. Also, I am located in Hawaii where the sun is intense and the electricity is expensive. Might turn out to be worth pursuing? Any new insights would be appreciated.

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