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Geoffrey Styles's picture
GSW Strategy Group, LLC

Geoffrey Styles is Managing Director of GSW Strategy Group, LLC, an energy and environmental strategy consulting firm. Since 2002 he has served as a consultant and advisor, helping organizations...

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How Green Is My Electric Vehicle?

One of the biggest challenges in assessing the environmental benefits of electric vehicles is that electricity is generated in so many different ways, with differing costs and consequences, and that patterns of generation vary by region, season, and time of day. As a result, categorical claims that EVs are always greener than the hybrids against which they compete most directly, or even compared to efficient non-hybrid compact gasoline or diesel-powered cars, must be suspect. The Union of Concerned Scientists (UCS) has just issued a report that takes some of the mystery out of such comparisons, including a helpful map showing likely EV emissions expressed in terms of equivalent miles per gallon from a gasoline vehicle. The takeaway is that as of now, the emissions advantage of purchasing an EV depends heavily on where you live, with equivalent emissions from average grid power in many parts of the country about on a par with those from a small car like the Chevrolet Cruze, and not even as good as from a Prius-type non-plug-in hybrid.

This apparent paradox becomes clearer when you examine the cities map that the New York Times distilled from the report, reflecting the local basis of electricity generation. An EV operated in L.A. or San Francisco would unambiguously beat a Prius on emissions, while an EV in my neighborhood in Northern Virginia would have only a slight edge, and one in Denver would yield emissions comparable to an ordinary car getting 33 mpg, unless the owner was scrupulous about recharging only when greener power was available. That’s because despite the declining share of coal-fired power in our national generation mix, there are still many regions and locales where coal dominates the grid, and the emissions from coal-fired generation are considerably higher than from natural gas or low-emission nuclear and renewables.

Any report such as this must incorporate a number of assumptions, and from my fairly quick perusal of the details they seem generally well-identified here. The UCS’s emission-equivalent miles per gallon calculation is based on a Nissan Leaf getting 3 miles per kilowatt-hour (kWh.) Grid emissions are calculated using a model of average hourly emissions over the course of the year. It didn’t appear that these hourly-averaged figures were weighted for seasonal variations in driving patterns, but that’s probably more nuance than is necessary at this level of scrutiny.

The report also includes information about recharging costs in different locations under different rate plans. Prospective EV buyers would benefit from taking the time to understand what these issues mean in their specific locations before investing in one. From my perspective, the report should also provide serious food for thought for policy makers concerning the wisdom of a single federal tax credit for EV purchasers in the US. As hard as that policy is to justify in the best of locations, based on the equivalent cost per ton of CO2 avoided, it looks positively senseless in locations where coal is still king. And while the report makes the point that the generation mix in many regions will become cleaner over time as utilities respond to renewable portfolio standards and other policies, buying an EV in a high-emissions region and counting on that factor to improve the car’s environmental benefits during its lifetime seems like a risky bet, particularly in economic terms.

The biggest caveat I’d offer concerning the report is its emphasis on comparing EVs to non-hybrid compact cars, both on costs and emissions. That just doesn’t seem realistic, given the array of choices and types of consumers in the market. While the number of consumers willing to consider an electric vehicle is increasing, the “take rate”–the number who actually convert their interest into a purchase decision, remains minuscule, resulting in sales of just 0.3% of all US cars sold in March. Meanwhile hybrids have benefited from rising gas prices to hit 3.4% of sales. It’s also worth recalling that the fuel, emissions and dollar savings from improved fuel economy decline with each additional increment. Hybrids already capture the most valuable savings over conventional cars, while the incremental fuel savings from stepping up from a hybrid to an EV are roughly comparable to what hybrids achieve, but require additional battery capacity and electricity, neither of which is free. That makes hybrids the technology that EVs must beat. As helpful as the information provided in the UCS report should be for consumers, the ultimate decision to buy an EV seems driven more by values than value, at least until EV costs fall significantly.

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douglas card's picture
douglas card on Apr 18, 2012

2012 Camry Hybrid – 43mpg adn can defiintely get close to 50mpg after learning to hypermile.  This is based on my 09 Camry getting 39mpg now after driving it over 2 years. 

Nathan Wilson's picture
Nathan Wilson on Apr 19, 2012

Geoffrey, your preference for comparing electric cars to hybrids (rather than conventional cars) makes a lot of sense.

However, considering the average emissions of the local electric grid is off the mark.  No one is building coal plants anymore.  So any new electrical demand will be supplied by gas (at least for evening charging).  Are them many places left where there is un-used night-time coal capacity?

Geoffrey Styles's picture
Geoffrey Styles on Apr 19, 2012


There are plenty of folks on this site with more power industry expertise than I have picked up by osmosis over the years, coming from the fuel side, so I would defer to them for anything definitive.  On a macro basis, particularly as the number of EVs grows, what you suggest makes sense.  However, the fact remains that they are today so few that their presence in most markets seems unlikely to cause the next generator to dispatch when they plug in.  Doesn’t that mean they take what’s already there when they plug in, which in a non-trivial number of cases will be generated from coal?  (Data centers are coming under scrutiny for the same issue, and they represent a much larger load than EVs today.) UCS looked at this on an hourly-average basis and concluded essentially that, if I read their report correctly. 

Ory Zik's picture
Ory Zik on Apr 19, 2012

You make some great points.  This UCS report makes two true statements: The MPG of an electric vehicle (EV) is better than that of an internal combustion engine vehicle (ICEV), and how environmentally friendly an EV is, depends on where it is getting its fuel.

Given this, the question then becomes, how can we maintain an intuitive understanding of an electric car’s “MPG” rating and yet take that understanding to the next level of accuracy?

Just as different ICEVs have different MPG ratings, different EVs have different efficiencies ranging from 2.5 to 4 miles per kilowatt-hour (kWh).

The disconnect comes when you try to ask the average person what it means to use a “mile per kWh” in electric fuel. In the U.S., we’re conditioned to understand what it means to measure miles per gallon, but when it comes to kilowatt hours…we’re far more likely to simply scratch our heads and shrug.

Different regions have different generation efficiencies. The following map shows those generation efficiencies in a metric of Electricity per Gallon of Gasoline, or EPG, measured in kilowatt hours per gallon of gasoline or Energy Points:

This helps provide the whole picture. If we are in California (EPG of 21) have a car running at 3 miles per kWh, our MPG is 3 times 21 or 63. If we drive the same car in Ohio (a coal state with an EPG of 11), our MPG becomes 33. Ultimately, with this equation, we are able to take our evaluations to the next level of accuracy to do an intuitive, side-by-side MPG comparison, to confidently say that the MPG of the EV is superior to that of than an ICEV and confirm that—yes—just how efficient it is depends very much on where you are located.

Rick Engebretson's picture
Rick Engebretson on Apr 20, 2012

A lot of opportunity in hybrid drive systems and fuels. CNG and battery chargers are drop-in capable now. Microturbines are interesting.

Geoffrey Styles's picture
Geoffrey Styles on Apr 20, 2012

Dr. Zik,

This seems useful, and I agree that the development of a meaningful and intuitive metric for EV end-use efficiency is important.  Where’ I’d differ is in your last sentence.  It’s quite clear that a number of non-plug-in models are more efficient in these terms than EVs in many parts of the country, and that while the power mix is evolving toward lower emissions the competitive bar from non-plug-in cars is also increasing.  80 mpg diesel hybrids have been demonstrated, and they may not be the final word on liquid fuel efficiency. 

Ultimately, from a policy perspective, it comes down to the goals we are pursuing.  If it’s saving oil, then EVs win in even the highest-emitting regions of the grid.  If it’s emissions, then it depends on location, as discussed.  In both cases, the cost of those savings of oil or emissions remains relevant, and today’s EVs are far from a low-cost solution when the cost of subsidies is included.  For consumers, the comparison is likely to have far more to do with the mix of cost and convenience that these vehicles deliver, and I fully expect that proposition to improve significantly over time.

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