Charging electric vehicles have been found to be more cost-effective at night, but researchers warn that this benefit comes with a trade-off: increased air emissions.

Opting to charge electric vehicles late in the evening is cost-effective because demand for energy is low, meaning electricity is cheap to generate, which grid operators prefer. However, researchers from the Carnegie Mellon University have discovered that late-night charging significantly increases greenhouse emissions, resulting in more air pollution compared to charging electric vehicles right away after a driver comes home.

For the study, published in the journal Environmental Research Letters, the researchers looked at changes in power plant operations depending on the load charging electric vehicles produced and projected emissions using models. They also factored in consequences of the increased air pollution on the environment and health.

According to their findings, the researchers saw that late-night charging results in a drop of up to a third for generation costs because demand is addressed by coal power plants. However, the shift to the cheaper energy source brings in additional greenhouse gas emissions that lead to 50 percent higher costs to the environment and human health.

Coal power plants don't operate at full capacity after the sun sets so they are ready to meet any nighttime demands for energy, like charging an electric vehicle. Alongside that, however, the power plants produce sulfur dioxide, which greatly affects health.

In a different study published in the journal Environmental Science & Technology, Jeremy Michalek and colleagues assessed greenhouse gas emissions produced by charging electric vehicles all over the United States and saw the same thing: charging late at night does increase levels of greenhouse gases in the air.

While this makes the use of electric vehicles appear counterproductive to green efforts and should definitely be considered, it does not worry the researchers too much as it is expected that many coal power plants will be retiring as a result of recent regulation.

The only trouble is that the Clean Power Plan has been halted by the Supreme Court while the initiative undergoes litigation, delaying the shift away from coal power plants.

The researchers recommend electric-vehicle users in coal-reliant areas like Philadelphia, Washington, D.C. and Chicago to not delay charging until later in the evening to help lessen greenhouse gas emissions.

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