Evidence suggests electric cars need night time charging

Researchers have found that ozone can be reduced when electric vehicle charging is done at night.

For those who don’t remember the ozone scares from the 90’s (Captain Planet), ozone is a known pollutant at low levels in the earth’s atmosphere. It can cause harm to respiratory systems and delicate plants.


Ozone forms in the air as hydrocarbons and nitrogen oxides react to sunlight. Two of the largest producers of these toxins are automobiles and electricity generating units (EGUs). You might be interested to know that some of the most densely populated areas in the US fail to meet the National Ambient Air Quality Standards even after 30 years of regulation.

According to the Institute of Physics, the popularity of Plug-in Hybrid Electric Vehicles (PHEVs) have risen in over the past decade because of cheaper fuel costs (versus gasoline), better efficiency, and positive influence on the environment thanks to fewer exhaust fumes.  

It is known that charging PHEVs at night is more cost-effective and reliable, but scientists publishing their work today, in IOP Publishing’s journal Environmental Research Letters, have found that charging at this time also results in lower levels of pollution. They measured this in an average across four cities and over the course of four representative modeling days.

PHEVs have the ability to run off battery power and gasoline. When PHEVs run off battery power they release no pollutants from their exhaust, however, the EGU’s, which provide electricity to charge batteries, do give off pollutants.

Based on this, researchers at the Massachusetts Institute of Technology (MIT) and the University of Texas modeled the outcome of replacing 20% of the vehicle miles travelled (VMT) by gasoline-run cars with PHEVs, using three different electric car charging situations to study the discharge of pollutants, specifically ozone, in the state of Texas.


There have been a lot of arguments over the best way to charge electric cars. The first set-up in this study was based on charging the car at off-peak times in the night. The second set-up involved charging to maximize battery life (charging just before use and only the amount of charge needed to complete the trip) and the third set-up involved charging the battery when it was a convenient time for the driver (typically just after vehicle use).


The outcomes of the study revealed that the overall levels of pollution, resulting from EGU discharges associated with charging, were lower than the level of pollution resulting from the emissions associated with 20% of gasoline VMT.


While night time charging was shown to yield the highest amount of nitrogen oxides, this led to the least amount of ozone on average across all cities and hours modeled as there is no sunlight for the emissions to react with. When the morning does come, the pollutants are spread out and diluted by other methods such as the wind.


The lead author of this study, Dr. Tammy Thompson of MIT said: “The results in general show positive air quality results due to the use of PHEVs regardless of charging scenario with the nighttime charging scenario showing the best results on average by a small margin.”


“This further supports efforts to develop regulation to encourage nighttime charging; an example would be variable electricity pricing. As more of the fleet switches over to PHEVs and a larger demand is placed on the electricity grid, it will become more important that we design and implement policy that will encourage charging behaviors that are positive for both air quality and grid reliability.”


And there’s good news for all you tree huggers, environment lovers and hybrid fans out there. The journal article from Environmental Research Letters is actually available to read, there’s no pay wall baby! Go and read it and soak up the information.