Charging an electric vehicle overnight produces 70 percent more emissions than if it were charged midday


Transportation-related emissions are increasing globally. Currently, light-duty vehicles – namely passenger cars, such as sedans, SUVs, or minivans – contribute about 20 percent of the net greenhouse gas emissions in the United States.

But studies have shown that switching out your conventional gas-guzzling car for a vehicle powered by electricity can make a significant dent in reducing these emissions.

A recent study published in Environmental Science and Technology takes this a step further by examining how to reduce the emissions associated with the electricity source used to charge an electric vehicle (EV).

Taking into account regional charging patterns and the effect of ambient temperature on car fuel economy, researchers at the MIT Energy Initiative (MITEI) find that the time of day when an EV is charged significantly impacts the vehicle’s emissions.

“If you facilitate charging at particular times, you can really boost the emissions reductions that result from growth in renewables and EVs,” says Ian Miller, the lead author of the study and a research associate at MITEI.

“So how do we do this? Time-of-use electricity rates are spreading, and can dramatically shift the time of day when EV drivers charge. If we inform policymakers of these large time-of-charging impacts, they can then design electricity rates to discount charging when our power grids are renewable-heavy. In solar-heavy regions, that’s midday. In wind-heavy regions, like the Midwest, it’s overnight.”

According to their research, in solar-heavy California, charging an electric vehicle overnight produces 70 percent more emissions than if it were charged midday (when more solar energy powers the grid).

Meanwhile, in New York, where nuclear and hydro power constitute a larger share of the electricity mix during the night, the best charging time is the opposite. In this region, charging a vehicle overnight actually reduces emissions by 20 percent relative to daytime charging.

“Charging infrastructure is another big determinant when it comes to facilitating charging at specific times—during the day especially,” adds Emre Gençer, co-author and a research scientist at MITEI.

“If you need to charge your EV midday, then you need to have enough charging stations at your workplace. Today, most people charge their vehicles in their garages overnight, which is going to produce higher emissions in places where it is best to charge during the day.”

In the study, Miller, Gençer, and Maryam Arbabzadeh, a postdoc at MITEI, make these observations in part by calculating the percentage of error in two common EV emission modeling approaches, which ignore hourly variation in the grid and temperature-driven variation in fuel economy.

Their results find that the combined error from these standard methods exceeds 10 percent in 30 percent of the cases, and reaches 50 percent in California, which is home to half of the EVs in the United States.

“If you don’t model time of charging, and instead assume charging with annual average power, you can mis-estimate EV emissions,” says Arbabzadeh. “To be sure, it’s great to get more solar on the grid and more electric vehicles using that grid.

No matter when you charge your EV in the U.S., its emissions will be lower than a similar gasoline-powered car; but if EV charging occurs mainly when the sun is down, you won’t get as much benefit when it comes to reducing emissions as you think when using an annual average.”

Seeking to lessen this margin of error, the researchers use hourly grid data from 2018 and 2019 – along with hourly charging, driving, and temperature data – to estimate emissions from EV use in 60 cases across the United States.

They then introduce and validate a novel method (with less than 1 percent margin of error) to accurately estimate EV emissions. They call it the “average day” method.

“We found that you can ignore seasonality in grid emissions and fuel economy, and still accurately estimate yearly EV emissions and charging-time impacts,” says Miller. “This was a pleasant surprise. In Kansas last year, daily grid emissions rose about 80 percent between seasons, while EV power demand rose about 50 percent due to temperature changes.

Previous studies speculated that ignoring such seasonal swings would hurt accuracy in EV emissions estimates, but never actually quantified the error.

We did – across diverse grid mixes and climates – and found the error to be negligible.”

This finding has useful implications for modeling future EV emissions scenarios. “You can get accuracy without computational complexity,” says Arbabzadeh. “With the average-day method, you can accurately estimate EV emissions and charging impacts in a future year without needing to simulate 8,760 values of grid emissions for each hour of the year.

All you need is one average-day profile, which means only 24 hourly values, for grid emissions and other key variables. You don’t need to know seasonal variance from those average-day profiles.”

The researchers demonstrate the utility of the average-day method by conducting a case study in the southeastern United States from 2018 to 2032 to examine how renewable growth in this region may impact future EV emissions. Assuming a conservative grid projection from the U.S. Energy Information Administration, the results show that EV emissions decline only 16 percent if charging occurs overnight, but more than 50 percent if charging occurs midday. In 2032, compared to a similar hybrid car, EV emissions per mile are 30 percent lower if charged overnight, and 65 percent lower if charged midday.

The model used in this study is one module in a larger modeling program called the Sustainable Energy Systems Analysis Modeling Environment (SESAME). This tool, developed at MITEI, takes a systems-level approach to assess the complete carbon footprint of today’s evolving global energy system.

“The idea behind SESAME is to make better decisions for decarbonization and to understand the energy transition from a systems perspective,” says Gençer. “One of the key elements of SESAME is how you can connect different sectors together – ‘sector coupling’ – and in this study, we are seeing a very interesting example from the transportation and electric power sectors.

Right now, as we’ve been claiming, it’s impossible to treat these two sector systems independently, and this is a clear demonstration of why MITEI’s new modeling approach is really important, as well as how we can tackle some of these impending issues.”

In ongoing and future research, the team is expanding their charging analysis from individual vehicles to whole fleets of passenger cars in order to develop fleet-level decarbonization strategies. Their work seeks to answer questions such as how California’s proposed ban of gasoline car sales in 2035 would impact transportation emissions.

They are also exploring what fleet electrification could mean – not only for greenhouse gases, but also the demand for natural resources such as cobalt – and whether EV batteries could provide significant grid energy storage.

“To mitigate climate change, we need to decarbonize both the transportation and electric power sectors,” says Gençer. “We can electrify transportation, and it will significantly reduce emissions, but what this paper shows is how you can do it more effectively.”

The transition to electric vehicles (EVs) is underway. While the transformation from conventional passenger vehicles (those with internal combustion engines, powered by oil-based fuels) to EVs (4-wheel plug-in vehicles propelled totally, or in part, by electricity [1]), is certain, the trajectory of this transition is far from settled.

EVs have a long history, and were quite prominent at the beginning of the automotive era [2]. The technology succumbed, however, to the business model set in motion by Henry Ford for the conventional passenger vehicle, and has lingered in the shadows ever since. Reasons for its dormancy are not only technical but the fact that it never, until now, engaged a supporting socially-constructed worldview that would create a powerful incentive for its ascendency [3].
The difference now is that both profit-making firms and countries perceive a commercial interest in EVs, and, most critically, EVs are perceived by larger society as a key force for combating climate change. Decarbonization strategies typically include the transition to electric transportation as a key element in long-term plans [4,5].

Specifically, the spark for a new transportation era has been provided by two entities: the audacious and improbable rise of the EV maker, Tesla [6], and the ascendency of China, and its bevy of EV startups, as the global EV hotspot. Over half of all EVs sold in 2018 and 2019, were sold in China, which perceives a new and enormous commercial opportunity arising from the transition [7].
Though understandably slower out of the gate, conventional automakers have now announced ambitious plans for electrification of their respective fleets, some of which are outlined in Table 1.

Dozens of EVs have already reached the market and over seven million are on roads across the globe today [8]. Yet sales are still exceedingly small relative to the overall market for passenger vehicles. In 2019 [9], sales of EVs totaled 2.2 million, just a 2.5% share of the market—meaning that only 1 in 40 passenger vehicles sold was an EV.

Some countries, such as Norway, Iceland, the Netherlands, and Sweden, sold more than 10% EVs in their respective markets [8] (Norway even had sales exceeding 50%). Yet the relatively small populations in these countries means that these sales hardly impacted global automotive markets in any meaningful way. It is safe to say, therefore, that the transition to EVs is in a very early stage. One way to think about this state of affairs is through the standard Rogers product diffusion model [10],
as shown in Figure 1.

With sales of 2–3 percent, EVs are still in the beginning stage of diffusion, attracting only the “innovator” or the beginning stages of the “early adopter” segments within a given population. Individuals within these segments of the population differ from those within the more general population. They are open to experiencing new technologies, are willing to overlook imperfections in the technology in order to achieve personal or societal goals, and are not swayed by mass marketing.

Numerous projections for when (or if) EVs will move up the diffusion curve have been attempted. A recent study that compared these projections in terms of the percentage of EVs sold twenty years from now (2040), revealed the range of these projections were from 10 percent to 70 percent of total market share [11]. This clearly shows that there is no consensus or conventional insight into how swiftly EVs will come to dominate the passenger car market.

The absence of consensus is a reflection of the contradictory forces now at play. While the author believes a quick transition to EVs is paramount for dealing with the major public policy issue of our time, climate change, there are no guarantees that a quick pace will come to pass. The pace is a product both of technological progress and societal reckoning with the challenge of climate change. This article reveals the major factors at play in determining the pace of the transition. Subsequent sections will outline why EV adoption rates are meager to date and the forces at work to both advance sales and retard them.

At the current time, strong governmental support for EVs is propelling sales, but ultimately the pace of this transformation will depend upon more favorable market forces.

Societal Impact
The underlying case for EVs rests on the fact that CO2 emissions from the transportation sector are large and growing faster than emissions from other sectors. Emissions are directly attributable to the carbon-emitting fuels powering the sector, namely, the oil-based products gasoline and diesel. In 2018, the transportation sector was responsible for approximately 14 percent of all Greenhouse Gas Emissions (GhGs) and a quarter of the emissions derived from burning fossil fuels [12].

Passenger vehicles are responsible for a large part (72%) of the sector’s emissions and are the primary reason for the sector’s rising emissions [12]. Fuel substitution, therefore, is a critical element in reducing the climate impact emanating from the transportation sector.

The impact of fuel substitution can, of course, be complemented by increasing fuel efficiency, car sharing, public transportation, etc. However, the ubiquity of personal ownership and operation of automobiles is all-pervasive (some have claimed we have created an “auto-centric” society based on auto-mobility) [13] and seems unlikely to diminish any time soon. The number of automobiles on our roads reached 1 billion in 2010 and now stands at 1.25 billion. Some projections now foresee a total of 2 billion autos on our roads by 2035 [14].

Second, it is now well established that EVs make a considerably smaller CO2 impact, across the vast majority of countries, than conventional passenger vehicles [9,15,16]. How much cleaner EVs are is a complicated calculation involving several variables, including EV and conventional vehicle operating efficiencies, and carbon levels of the electricity being delivered to EVs (as well as carbon levels of refined oil products).

The mix of primary fuels in the production of electricity varies from country to country and city to city within countries. The International Energy Agency (IEA) has calculated that the current average carbon intensity of global electricity is 518 gCO2/kWh, a quantity at which typical EVs are cleaner than most conventional vehicles [9]. Again, this average does not account for the large variation in carbon-intensive electricity across countries, such as the variation between Poland and Norway.

The good news is that with the expected greater use of renewable energy in the production of electricity in the future, EVs will become even cleaner over time. Developments in the power sector, therefore, will have a direct bearing on the societal benefits we can expect from the EV transformation within the transportation sector.

As the electric grid becomes cleaner, EVs will also make a significant contribution to reducing local air pollution, which remains the deadliest of all impacts resulting from the burning of fossil fuels [17]. Once again, the calculation of the EV impact on reducing local pollution is complicated.

The absence of tailpipe pollutants in vehicles traversing dense, urban populations is an obvious benefit. Yet, it if means the production of more dirty electricity at the power plant, and if the plant’s emissions drift into these dense urban conclaves, little positive air pollution results will be achieved. Site-specific analysis, therefore, is necessary to determine the extent of air pollution benefits derived from the EV transformation.

Perhaps the best way of thinking about global and local air pollution impacts of the EV rollout, therefore, is to recognize that its significance is dependent upon synergistic and interconnected improvements in electricity generation.

Still another benefit of the EV transformation will be the reduction in oil imports resulting from diminishing gasoline and diesel sales [12]. Large-scale oil production is centered in relatively few countries, some of whom are politically unstable. This instability creates conditions leading to price fluctuations and supply insecurities. Electricity, in contrast, can often be produced from domestic resources, particularly if these resources are renewable. Backing oil out of the energy supply system, therefore, can lead to greater price and supply security.

In summary, there are good reasons to believe that the movement to EVs will result in significant societal benefits. Having said that, however, it is important to remember that unlike “innovator” or “early adopter” segments of the population, the broader public will generally make automobile purchases not on the basis of achieving societal goals but rather according to personal taste.

Owners want to believe that there are specific advantages that accrue to them alone through the purchase of one vehicle model as opposed to another. These perceived advantages may be functional (such as affordability, reliability, and comfort) or may be symbolic (indicative of social status or group membership) [18].

It is important to point out, therefore, that there are already some functional and symbolic advantages of EVs that will attract the general population. In the section that follows, the personal benefits described pertain to owners of fully-electric vehicles (BEVs). Those owning hybrid plug-in vehicles (PHEVs) will, of course, see fewer benefits from those described.


  1. Union of Concerned Scientists. What Are Electric Cars? Available online: what-are-electric-cars (accessed on 17 July 2020).
  2. Cutcliffe, S.H.; Kirsch, D.A. The electric vehicle and the burden of history. Environ. Hist. 2001, 6, 326–328.
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    2009, 27, 411–427. [CrossRef]
  4. Victor, D.; Geels, F.; Sharpe, S. Accelerating the Low Carbon Transition; Brookings Institution: Washington, DC, USA, 2019.
  5. Project Drawdown. The Drawdown Review: Climate Solutions for a New Decade; A publication of Project Drawdown: San Francisco, CA, USA, 2020.
  6. MacDuffie, J. The Future of Electric Cars is Brighter with Elon Musk in It. 2018. Available online: https://www. (accessed on 27 April 2020).
  7. Kennedy, S. China’s Risky Drive into New-Energy Vehicles; Ctr Strat. & Int. Studies: Washington, DC, USA, 2018.
  8. Global BEV & PHEV Sales for 2019. Available online: (accessed on 27 April 2020).
  9. International Energy Agency. Global EV Outlook. Available online: outlook-2019 (accessed on 17 July 2020).
  10. Rogers, E. Diffusion of Innovations, 3rd ed.; Simon & Schuster: New York, NY, USA, 2003.
  11. Kah, M. Electric Vehicle Penetration and Its Impact on Global Oil Demand: A Survey of Forecast. Trends; A publication of the Columbia Center on Global Energy Policy: New York, NY, USA, 2019.
  12. Wang, S.; Ge, M. Everything You Need to Know About the Fastest-Growing Source of Global Emissions: Transport; World Resources Institute: Washington, DC, USA, 2019.
  13. Urry, J. The system of automobility. Theory Cult. Soc. 2004, 21, 25–39. [CrossRef]
  14. Voelcker, J. Two Billion Vehicles Projected to be on Roads by 2035. Green Car Reports. 2014. Available online: on-roads-by-2035 (accessed on 27 April 2020).
  15. Knobloch, F.; Hanssen, S.V.; Lam, A.; Pollitt, H.; Salas, P.; Chewpreecha, U.; Huijbregts, M.A.J.; Mercure, J.-F. Net emission reductions from electric cars and heat pumps in 59 world regions over time. Nat. Sustain. 2020, 3, 437–447. [CrossRef] [PubMed]
  16. U.S. Department of Energy. Emissions from Hybrid and Plug-In Electric Vehicles. Available online: https:
    // (accessed on 21 April 2020).
  17. Schnell, J.L.; Naik, V.; Horowitz, L.W.; Paulot, F.; Ginoux, P.; Zhao, M.; Horton, D.E. Air quality impacts from the electrification of light-duty passenger vehicles in the United States. Atmos. Environ. 2019, 208, 95–102. [CrossRef]
  18. Axsen, J.; Sovacool, B.K. The roles of users in electric, shared and automated mobility transitions. Transp. Res. Part. D Transp. Environ. 2019, 71, 1–21. [CrossRef]

More information: Ian Miller et al. Hourly Power Grid Variations, Electric Vehicle Charging Patterns, and Operating Emissions, Environmental Science & Technology (2020). DOI: 10.1021/acs.est.0c02312


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