CO2 payback of solar panels and an electric car

There have been a lot of studies and comparisons on the savings of CO2 emissions when pv solar panels are used in place of electricity from a utility company; however, since different solar panel manufacturers use different manufacturing techniques, and different kinds of power plants produce differing amounts of CO2, the results of these comparisons can vary widely. Different authors have in fact come up with different answers to this question, ranging from less than a year to, on the very high end, twenty five years before solar panels have offset as much CO2 as would have been produced by conventional means. With such widely varying results, it is difficult to draw any meaningful conclusions on the CO2 savings of solar panels by aggregating the available data. (See the survey article on this subject at http://energybulletin.net/node/17219 for details.)

We wanted to ask this question another way.  Since we purchased our solar panels in order to power our new electric car, we wondered how many miles of driving would it take to offset the full CO2 produced in the manufacture of our specific panels, and about how long will it take for that to happen?  By focusing on a very specific scenario, we expected that we could get a more meaningful result by basing our calculations on data that matched our usage exactly.

Sun Power, the manufacturer of our solar panels, has been forward-thinking enough to actually measure and keep track of the CO2 output of its global operations.  For 2009, they announced that they produced 328 metric tons of CO2 per MW of solar cells produced.  That sounds like a lot, but let's see how that compares to the CO2 emissions of the car I replaced with our Leaf.  Here are the calculations we did.

Wow! If I had kept driving my old car, it would have only taken 4,472 miles of driving to emit the same CO2 produced in the production of our solar panels.  I have had my Nissan Leaf for less than half a year, and I have already driven 6,218 miles in it.  By this simplified calculation, which accounts only for the miles driven and the knowledge that in the course of our first year with our solar panels, we will generate enough electricity to cover our consumption for both our car and our house, our solar panels are already carbon neutral.  Strictly speaking, we are still a little shy; we put our panels in during the winter, so our production has been a little under what it will average for the year.  So far, our panels have produced 1,232 kWh of electricity, whereas 4,472 miles of driving in my Nissan Leaf probably used about 1,400 kWh. Five months of production will be enough to get our solar panels to carbon-neutral, though -- pretty amazing, really.

There are some minor pluses and minuses to this equation that we did not account for.  On the minus side, I trickle-charge my car at work, which means that I really should add in extra CO2 from the coal and other assorted sources of electricity used for the portion of grid-sourced power that I put into my car.  Also, due to range issues, we cannot use the leaf for everything that we used the Integra for, so we will probably put another two or three thousand miles per year on our 20mpg Honda Element. On the plus side, our solar panels produce enough electricity to power my car and our house, so by the end of the year we will be gaining additional CO2 offsets from the power that we no longer need to purchase from the grid for home use. Finally, we have not accounted for the energy requirements to actually manufacture the Leaf itself; however, my previous car was fourteen years old and had 250,000 miles on it, so we consider that it is reasonable that we may have replaced it with a new ICE car at about this time anyway. In this sort of scenario, when you can offset the CO2 production of your solar panels solely from the savings of the miles-not-driven in your old car in less than half a year, that is a huge win for solar panels. Do the math for your particular situation, and see how your results compare; who knows, a solar-powered electric car may be in your future too!

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1. Conversion from metric tons CO2 to pounds CO2 from http://www.mtco2e.com

2. Pounds of CO2 produced from burning a gallon of gas from http://en.wikipedia.org/wiki/Greenhouse_gas (scroll down to "Relative CO2 emission from various fuels" for the reference -- sorry, there's no anchor on the page)