Electric Vehicles: Climate Saviors, or Not?

Where electric vehicles are located and when they are recharged matters more than you might think.

There are several reasons to support the growth of electric vehicles (EVs) worldwide, but one of the most compelling is its potential to reduce greenhouse gas emissions that emanate from petroleum-driven vehicles. Projections of the growth of EV use over the next 25 years vary widely, due largely to uncertainties about the development of batteries to propel the electrical systems. If new batteries can be produced that have significantly more energy density and at lower costs, the future of electric vehicles is bright. New EVs, such as the Chevy Bolt and Tesla Model 3, which are scheduled for market introduction over the next couple of years, provide some cause for optimism, but the road ahead is still uncertain.

Uncertain developments, however, need not prevent us from examining our current understanding of how EVs will affect climate change. Some EV makers (for example, Nissan, with its “zero emission vehicle” slogan for its Leaf model) want us to believe that since no emissions emanate from the car itself, there is no negative impact on the environment whatsoever. That notion has been widely debunked, and the only remaining question is how much of a climate impact the vehicles do have. Numerous studies have sought to carefully answer the question, but there is no simple answer.

My analysis here can be divided into three levels. The first is the international level, based on country-specific data. The major factor determining the scale of the impact from electric vehicles is the carbon intensity of a country’s electrical grid. The more carbon intensive the grid (primarily due to the burning of coal), the less effective EVs will be at reducing carbon emissions. The second level focuses on intra-country electricity-generation patterns. Few countries get their electricity from a single grid, so it is important to look at the variation across geographical scales. Studies done in Canada and the United States reveal considerable regional variation in electric-generation sources. The third level of analysis is the temporal dimension, which is the time of year and time of day that owners charge their vehicles.

If new batteries can be produced that have significantly more energy density and at lower costs, the future of electric vehicles is bright.

These analyses make it clear that the widespread introduction of EVs, by itself, is insufficient to lead to reduced carbon emissions from the transport sector. Electricity grids need to incorporate greater levels of clean, renewable energy. Equally important, and less understood, electricity providers must incentivize public recharging of EVs when renewable energy generation is at its peak. As will be seen, this is not currently the case, at least in the United States.

It is important to clarify which vehicles are considered EVs. In these analyses, EV is a generic term to describe both pure electric vehicles (often labeled Battery Electric Vehicles or BEVs) and plug-in hybrid electric vehicles (sometimes referred to as PHEVs). BEVs use batteries as the only source of power to the electric motor driving the vehicle. The Tesla is the best known example. PHEVs have both an electric motor and an internal combustion engine. PHEVs can be distinguished from hybrid electric vehicles (HEVs) by their ability to travel a significant distance on battery-generated power alone. HEVs, such as the well-known Toyota Prius, are not included in the definition of EVs because their battery systems are relatively small, require no owner plug-in to the electrical grid, and serve solely to assist the internal combustion system in obtaining better gasoline mileage rather than supplanting it.

The international picture

The global inventory of operating EVs, as of the end of 2015, totaled just over 1.25 million. The optimists point out that this represents a huge increase from the beginning of the decade, when there were virtually no EVs on the road and prospects for future growth were uncertain. The pessimists counter that although the number seems large in isolation, it still represents a miniscule portion of the personal vehicle inventory, no more than 0.1% of all such vehicles. Advocates of either perspective can agree that there is no hard evidence to support either outlook because we are still at the earliest stages of the technology adoption life cycle.

According to data published by the International Energy Agency (IEA), the growth of EVs globally has been quite uneven. Five countries (China, Japan, the Netherlands, Norway, and the United States) are home to about 1 million EVs, roughly 80% of the total. The United States is the largest home, with something more than 400,000 EVs. President Obama, at the beginning of the decade, had projected that the United States would have 1 million EVs on the road by the end of 2015, but he was clearly in the optimistic camp. Although the United States was the pioneer in EV development, ownership growth rates are higher in other countries. China, currently number two on the list of EV owners with more than 300,000 vehicles, will likely soon overtake the United States. The Chinese government is making a major push to increase EV ownership, offering multiple incentives for individuals and organizations to purchase such vehicles. The pace of EV ownership is also picking up in other countries, and the global total will likely exceed 2 million vehicles in 2017.

Knowing where these cars are helps calculate their impact on climate emissions. The IEA reports the carbon intensity of national grids in grams (g) of carbon dioxide (CO2) per kilowatt hour (kWh), which can then be used to gauge emission rates for that country’s EVs. The IEA has also calculated that when EVs receive electricity with emission levels exceeding 559 gCO2/kWh, they, unfortunately, are net contributors to climate change when compared with conventional vehicles. Other studies claim that the IEA’s calculation is too conservative, and that the threshold for climate improvement might be as high as 640 gCO2/kWh. For the purposes of simplicity, however, I will use the IEA number for comparative purposes.

The latest global data on the carbon intensity of electricity grids is from 2013. In general, carbon intensity has been declining, but the difference from 2013 will not be dramatic in most countries. The carbon intensity for the countries with the most EVs is a very mixed picture.


From Figure 1, we see that the use of EVs in the United States should, on average, have a slightly positive effect on reducing carbon emissions. And the 2013 level is an improvement from the 2009 level of 529 gCO2/kWh. With the significant post-2013 reduction in coal use, we can expect that the 2015 level will be even lower. On the other hand, Japan is moving in the opposite direction. With the closure of approximately 50 nuclear power plants in the wake of the Fukushima accident, EV use now marginally contributes to climate change.

The situation in China is worse. With coal comprising approximately 70% of China’s electricity generation, the carbon intensity of the grid is high. China has made bold climate pledges, but evidence on the ground today is mixed. It has the most ambitious renewable energy program of any country, but, at the same time, continues to build new coal-fired plants. The fact that China is likely to have more EVs than any other country by the end of 2016 provides little cause for cheer among those concerned about climate change.

Norway demonstrates the opposite extreme. The carbon intensity of its grid is extraordinarily low due to the overwhelming presence of hydroelectric power. Because of the government’s generous incentives for purchase, EV ownership per capita is higher than anywhere in the world. Approximately 30% of all new vehicles sold in Norway during the last quarter of 2016 were EVs.

At the international level, therefore, the evidence is mixed. In some cases, EVs reduce CO2 emissions, and in other cases, they actually result in more carbon emissions than would conventional vehicles. But if countries diligently work to decarbonize their electricity grids, the outlook will be much more promising than today’s evidence suggests.

A closer look within countries

Country-level numbers reveal that the carbon intensity of electricity varies widely among regions. Because EV ownership also varies significantly by region, the emissions impact of EVs could also vary by region. This can be seen most starkly in Canada.

In some cases, EVs reduce CO2 emissions, and in other cases, they actually result in more carbon emissions than would conventional vehicles.

According to the IEA, the carbon-intensity of Canada’s national grid is 158.42 gCO2/kWh, which is certainly cleaner than most national grids, due especially to the dominance of hydroelectric power. Four of the 10 Canadian provinces, however, generate their electricity from grids that produce carbon at about the 559 gCO2/kWh threshold for climate improvement; the other six provinces had emissions closer to Norway’s. The question is whether Canadian EVs are being driven in the cleaner or the dirtier provinces, and here there is good news. Approximately 95% of the EVs purchased to date are found in just three provinces—Ontario, Quebec, and British Columbia—all of which are low-emitting provinces.

In the United States, the rise of cheap natural gas is likely to continue to drive down the overall carbon intensity of grid electricity. Between 2013 and 2015, coal use dropped from 40% of all electricity generated to 34%, and plans for the next five years involve even more significant numbers of coal plants being shuttered. So the overall trend is positive.

As of 2013, there is considerable heterogeneity within states and regions with respect to the average carbon intensity of their electricity. Idaho has the cleanest electricity (153 gCO2/kWh) and Montana the dirtiest (1,018 gCO2/kWh). We have excellent data for EV sales and ownership, so it is not difficult to match EV sales to state carbon levels.

The top 11 states for EV sales possess 80% of all EVs. This finding is strongly influenced by the state of California, which contains nearly half (48%) of all EVs. There are multiple reasons for the preponderance of sales in California, including a temperate climate, generous purchase incentives, a liberal political philosophy, and the largest selection of EV models made available for purchase because of the state’s zero energy vehicle mandate, which pressures all automakers to sell clean vehicles. Fortuitously, California’s electric grid is one of the cleanest in the country. Overall, the top 10 cleanest states in the United States possess 60% of all EVs, while the bottom 10 possess only 3%.

In summary, an analysis of intra-country data provides a more positive picture of EV development than what we derive from country data. But we need to drill even deeper to acquire a fuller picture, and we do so by examining temporal issues.

Time matters

The data and findings presented thus far have been based on average CO2 intensity numbers. We know, however, that releases of CO2 to the atmosphere from EVs are dependent on the season of the year and the time of day EV batteries are recharged. Several studies have attempted to account for these factors.

EVs do not perform as efficiently in cold weather as they do in moderate temperatures. This effect is, in part, the direct impact of cold temperatures on battery performance and, in part, the need to provide heating for the vehicle occupants. In conventional automobiles, engine heat is used to warm the inside of the car. In EVs the battery must be used to produce heat, which it usually does through inefficient resistive heating. We would expect, therefore, that EVs in the northern US states would require more electricity than average to operate (though conventional automobiles also suffer efficiency losses in colder climes). As noted previously, since nearly half of all EVs today operate in the primarily moderate climate of California, the overall losses due to seasonal variations are not major.

The more serious temporal issue has to do with the times of day when EV owners recharge the car’s batteries. Utilities use different sources of energy during the day and night to produce the electricity that reaches consumers, and these patterns of generation are not random. To achieve maximum carbon reductions, it is important, therefore, that EV owners charge their cars at times when low-carbon electricity sources are being used. Using average emission factors does not capture this important temporal dimension.

Energy analysts seek to account for this factor using what is termed “marginal emissions intensity,” that is, the likely CO2 emissions produced when electricity demand from EV recharging is added to the grid. In effect, it is the matching of EV recharging with the kind of electricity being produced at that time. Although not random, marginal emissions are not easy to calculate because each utility is unique, having its own basket of resources available to produce electricity. Nonetheless, important generalizations can be made with respect to EV recharging patterns and the marginal emissions that result.

We know, for example, that roughly 85% of all recharging takes place at home and this is usually done overnight. EV owners use as much as four times more electricity during the night than do typical Americans.

Utilities tend to encourage EV owners to charge overnight since it has capital infrastructure that is not being used then. Approximately 30 US utilities now have special nighttime rates for EV owners. For example, EV owners and other customers of Georgia Power can obtain rates as low as 1.4 cents/kWh if they charge during the nighttime; they are billed 20.3 cents/kWh for charging during peak electricity periods. Consequently, it is a win-win proposition for the consumer and the utility. The consumer takes advantage of very inexpensive electricity and the utility gains revenue from otherwise downtime operations while reducing stress on the grid system during times of peak demand.

EV owners use as much as four times more electricity during the night than do typical Americans.

Unfortunately, these patterns adversely affect emissions. A number of studies examining overnight recharging and marginal emission factors have concluded that this practice produces higher than average CO2 emissions and, when combined with colder temperatures, may make EV operation in the upper Midwest a net contributor to CO2 emissions. Even in relatively clean states, such as California, the difference in CO2 emissions from nighttime to daytime can be significant. The nighttime start-up of coal plants in response to the additional electricity load from EVs increases marginal emissions. And, of course, clean solar energy is not available at night. As one study has explicitly stated, there is a “fundamental tension between electricity load management and environmental goals.”

From analysis to practice

We need a merger of new policies and advanced technology to resolve this conundrum. Certainly the growth of renewable power in electricity generation provides an opportunity to synchronize battery recharging with clean power. Regulatory agencies and major power producers need to be cognizant of these new resources coming on line and restructure rates to promote recharging during peak periods. Where solar energy is plentiful, providing incentives for workplace recharging would make sense. In some cases, such as with the prodigious amount of nighttime wind power in Texas, overnight recharging could remain the option of choice.

Greater public resources need to be devoted to large-scale electricity storage and to making new forms of EV recharging possible. With the advent of large-scale electricity storage, it may be possible in the future to completely sever the temporal link between generation and consumption. Storing clean energy and later releasing it at will could make EV use a truly clean enterprise. Even before the advent of large-scale electricity storage, new forms of charging EVs could be encouraged, such as wireless or inductive recharging that would give EV owners more flexibility in when they charge. These forms of recharging hold the potential for entirely new recharging behavior, which would make EVs a feasible option for many more drivers.

Governments and electricity generators can also promote the distribution of innovative recharging stations that can, through the advancement of sophisticated software, source the cleanest electricity production in real time and configure the recharging process on that basis. Start-ups such as WattTime and eMotorWerks are pioneering this technology, which could reward consumers for drawing on the cleanest electricity possible. Sonoma Clean Power, for example, has agreed to distribute 1,000 of these charging stations (called JuiceBox Pro) free of charge to its electricity customers and EV owners.

In summary, currently EVs can be climate positive or climate negative depending on where they are located and when they are recharged. As demonstrated in this analysis, EV ownership is taking place in some regions where there is a concurrent growth in the use of renewable energy. That is the good news. As EVs grow as a percentage of sales, however, they will inevitably spread to regions with grids that are more carbon intensive. This is the story right now in China, where the electric grid is still very carbon intensive.

In a world that needs to quickly reduce its greenhouse gas emissions to avoid the worst consequences of climate change, the transport sector remains, perhaps, the most significant technical challenge. The promise of EVs points to one important avenue of progress, but as I have shown, rising EV sales must be synchronized with renewable energy growth. As well, conscious efforts must be made to synchronize battery recharging with the temporal peculiarities of renewable generation, which themselves will vary from region to region. The advent of electricity storage may be the ultimate answer to this need, but until storage becomes economically viable, the challenge is to find ways to encourage recharging when renewable sources are providing the power.

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Cite this Article

Barkenbus, Jack. “Electric Vehicles: Climate Saviors, or Not?” Issues in Science and Technology 33, no. 2 (Winter 2017).

Vol. XXXIII, No. 2, Winter 2017