Post #1150: Rapidly dropping carbon intensity of electrical generation in Virginia

Posted on May 23, 2021

This post is about electric cars.  And about what has and hasn’t changed since I bought a new car back in 2005.

The 2005 automobile market, and a no-brainer decision.

My wife and I drive around in a very beaten-up 2005 Prius.  We bought that new, in 2005, because it was head-and-shoulders more efficient than any other gasoline-powered car on the market.  Here’s a graph showing that, based on my calculation from EPA mileage and other data.   This is literally the analysis that I based my purchase decision on.

At that time, if you were going with gasoline as your fuel, and wanted an efficient car, the choice of the Prius was a no-brainer.  For me, anyway.  The only thing even close to the efficiency of a Prius was a Honda Civic Hybrid, and I just plain couldn’t fit into one of those.  So the Prius was it.

Remember, this is four or five years before the first Tesla was sold.  Electric wasn’t an option.  If you wanted an efficient car, and weren’t fond of the dirty diesels of that era, and you wanted efficient transport, there was one clear choice.

Almost twelve years ago, we had a 5KW rechargeable battery pack installed (brand name Hymotion).  This converted our standard Prius to a plug-in Prius, circa 2009.   Much of the energy to propel the car (around town, at least) comes out of the wall socket, not the gas tank.

That Hymotion system used one of the first long-lived exotic-chemistry lithium batteries.  These use a combination of lithium, iron, cobalt, phosphorous and other materials to prevent the rapid deterioration that occurs in standard rechargeable lithium-ion batteries.  In theory, this raises the longevity of the battery from a few hundred cycles to thousands of charge/discharge cycles.  Hymotion claimed that the battery would retain 80% of original capacity after 7000 charge-discharge cycles, or about 20 years of daily discharge.  More than a decade later, I’d say there’s a good chance it’ll do just that.

And that 15-year-old technology is still pretty hard to beat.  The snapshot above shows the mileage we were getting in that 2005 Prius.  It’s the mileage for the current tank of gas, as of yesterday.  To be clear, we’ve been relying on the electrical side more, in light of the current (short-lived) gas shortages.  That said, at 82+ MPG, that’s respectable even compared to modern cars.

We’re still driving that aged car because there still isn’t a gasoline-powered plug-in vehicle that’s substantially more efficient.  (Well, that, and the fact that I’m too cheap to buy a new car while the current car is still running.)  We could do slightly better with a brand-new Prius Prime (the plug-in version of the Prius).  But only slightly.

Same car, same mileage, lower carbon footprint.

I wasn’t thrilled about buying that aftermarket plug-in battery pack for the Prius.  When I did the math, at that time, running on electricity was only a modest improvement over running on gasoline, in terms of carbon footprint.  It was, as I recall, about 30% carbon-sparing.  Every mile driven on electricity produced 70% of the C02 of a mile driven on gasoline. 

That was due in part to the incredible efficiency of the Prius gasoline engine.  If you were to do that same calculation, at that time, for a standard gas engine vehicle, the C02 savings would have been vastly higher.

Even now, most people completely misunderstand why the Prius was so efficient.  It had almost nothing to do with the electrical side of the car.  Almost all the efficiency gains came from using a different type of internal combustion engine — an Atkinson-cycle engine — instead of the standard Otto-cycle engine that powers almost every other vehicle on the road.  In the Atkinson-cycle engine, the power stroke is much longer than the compression stroke, allowing the car to extract more energy from a given amount of gasoline.  This makes it far more efficient than the standard Otto-cycle engine (where compression and power strokes are of roughly equal length), but at a cost of a very poor weight-to-power ratio.  The entire electrical side of the car was put into place to give the car adequate on-road performance despite the use of the low power-to-weight Atkinson-cycle engine.

But also due to the the relatively high C02 emissions per KWH for electricity generated in Virginia.   At the time, electricity generated in the Virginia region of the grid generated about 1.1 pounds of C02 for every kilowatt-hour produced.  (There are, of course, nuances there, but that’s the average, and that’s arguably the right figure to work from).  At that rate, charging the car out of the wall socket, and driving on electricity, produced about 70% as much C02 as driving the car with gasoline.

There were, to put it mildly, much cheaper ways to reduce my carbon footprint than buying a big, expensive aftermarket battery for my car.  So, while it provided a net reduction in carbon footprint (after factoring in the energy required to make the battery), that came at a high cost per pound of C02 emissions avoided.

Now that I’m thinking about new cars, I decided to redo that calculation.  So I looked up the most recent data on C02 emissions and electrical generation.  And my, how the world has changed.

Based on the most recent data from US Energy Information Administration,  Virginia now produces just 0.68 pounds of C02 per KWH generated.  That’s as of 2019.

You have to be careful when you see something like that.  Maybe there was a change in methodology.  Maybe the older and newer data aren’t comparable.  So I need to find a time-series on that figure, all done under one method.  Turns out, under the link for full data tables, you can find all that information for the past two decades, for Virginia.

Here’s the carbon-intensity of Virginia electricity, for the past couple of decades, ending in 2019.  Near as I can tell, this is due almost entirely to the substitution of natural gas for coal, and only to a small degree to the substitution of renewables for fossil fuels.

Source:  US EIA, data for Virginia, full tables 1-16, Table 7.

Apparently this is happening pretty much all across the U.S.

And so, when I redo my calculation, I find that electrically-powered miles now produce 61% less C02 emissions per mile, relative to gasoline-powered miles, in my ancient Prius.   And bear in mind, that’s against the high efficiency standard of the Prius Atkinson-cycle engine.  If you’re driving around in an old-fashioned non-hybrid car, the C02 reduction would be much more than that.

And I didn’t have to lift a finger to get that improvement.  It’s not that my gas engine has gotten any dirtier.  It still ranks close to the best you can buy.  It’s that the Virginia generation mix has rapidly shifted to far less carbon-intensive sources of power.


When I went car-shopping back in 2005, the decision to buy a Prius was more-or-less a no-brainer.  Not only was it just about the cheapest car you could buy in terms of total cost of ownership, it was far-and-away the most efficient.  And, at that time, that meant it had far and away the lowest carbon footprint per mile.

(There’s a little caveat there, in that the production of the Prius is modestly more energy-intensive than a normal car, owing mostly to the large amount of copper contained in the roughly 40 HP worth of electric motors embedded in the transmission.  Based on Toyota’s detailed analysis, relative to a non-hybrid car of the same size, that excess energy cost is paid back in the first two years of normal operation.)

Based on the same criteria I used back in 2005, it’s starting to look like electric propulsion is the new no-brainer.  Obviously, I’m going to have to do a bit more homework.  But the rapid de-carbonization of the electrical grid has shifted my opinion about the benefits of electrical propulsion relative to gasoline propulsion.  Even with an efficient vehicle like the Prius, electrical propulsion now has a vastly lower carbon footprint per mile (at least, here in Virginia, under our current electrical generation mix).

So, where I was almost indifferent to the benefits of electrical transport a decade ago, I really cannot in good conscience ignore the benefits now.  The per-mile carbon footprint of electrically-driven transport is vastly lower than it was the last time I looked at this (circa 2009), and appears to be continuing to drop rapidly.

For sure, my next car will at least be a plug-in hybrid, so that local trips can be done purely on electricity.  And maybe it’s time to take the plunge and go for an all-electric vehicle.

I realize all you Tesla/Leaf/Bolt drivers out there are probably laughing at my hesitancy.  But you have to realize, the last time I got serious about this decision was 2005, and I haven’t really had any regrets since.  And it’s not like the emissions of my current car will magically disappear when I buy a new one.  Somebody’s going to continue to drive that old Prius, and for now, I might as well do that.  But when the time comes to scrap that car, I want to have a firm opinion about what my next vehicle should be.  And right now, electric is looking pretty good.