Electric vehicles aren’t 100% clean. That electricity charging your car comes from a source. How the power is generated at that source determines how clean or dirty the electricity you use really is. Now researchers at the MIT Energy Initiative find that what time during the day your Tesla or Bolt is charged makes a difference in how much it really is polluting. 

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

The best time to charge your EV is midday for sunny climes and overnight in the Midwest

So the short answer to when the best time is to charge your EV is midday for sunny climes and overnight in the Midwest. The research found that in places like California, charging overnight is 70% dirtier than charging midday. That’s because solar energy powers the electric grid throughout the day. 

For New York, charging your EV overnight reduces emissions by 20% over daytime charging. That’s because nuclear and hydropower make up the majority of power on the grid overnight. “Charging infrastructure is another big determinant when it comes to facilitating charging at specific times — during the day especially,” says Emre Gençer, also 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. This is going to produce higher emissions in places where it is best to charge during the day.”

“It’s great to get more solar on the grid and more electric vehicles using that grid,”

“To be sure, it’s great to get more solar on the grid and more electric vehicles using that grid,” says Maryan Arbabzadeh, a postdoc at MITEI. “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.”

Some of what the researchers are doing is to project how California’s ban on gasoline-powered vehicles by 2035 will affect carbon emissions. One part of this is could fleets of electric vehicles be used to store grid energy? Large electric vehicle fleets could be used for energy storage when they are down overnight and during a weekend.