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When should you charge your electric car?

When should you charge your electric car?

By many measures, the American life is electric. In the morning, as we plunk down at our desks, we plug in our laptops. At night, we plug-in our phones before we go to bed. Now that electric cars are hitting the streets, when people get home in the evening, an increasing number are plugging in their cars. And that concerned several utility executives who spoke at the Plug-in 2013 conference in San Diego this month.

To be clear, in follow-up interviews, the executives said there’s no reason to worry about charging electric cars now, despite the challenges the grid has faced during summer heat waves in recent years. But looking forward to 2020 or 2030, several power-company representatives said their worst nightmare was to have swarms of electric cars arrive home at 7 p.m. and promptly plug in to 240 volts.  Most of the utilities represented said their peak load times ran from early afternoon until 11 p.m.  Electricity demand tapers gradually before then, but several utilities showed graphs that demonstrated a steep, and not unexpected, drop-off late in the evening. While the more gradual earlier tapering allows room for some charging, it doesn’t allow room for major spikes in demand.

So power companies are working on developing rate structures to encourage electric car drivers to begin charging later. These could include time-of-use rates that remain high through 11 p.m. (Relatively few utilities currently have time-of-use rates, because they are expensive for them to implement, and they are not widely needed yet.) In the meantime, it is helpful for the utilities if electric car drivers stagger charge times, rather than all plugging in at once.

One way to help ensure a gradual increase in electricity demand is for drivers to use the charging timers and smart-phone app connections already built into electric cars. Ideally, these should allow you to set the time when you plan to leave in the morning and want the car to be fully charged, rather than just the time you want the charge to start. That way, if the car will be plugged in for longer than it needs to get a full charge, it can start charging later, or start more slowly, and ramp up as other electricity demand goes down.

Some of these apps, such as those in the Chevrolet Volt, allow you to program your utility’s rates into the system, and tell it what rate you’re willing to pay to charge. In the future, the cars should be able to download your rates directly from the utility.

A simpler solution may be the new wall chargers from Siemens and others with built-in delay buttons. But for those to work, consumers need to be well versed in how much power they need, when they need it, and how fast their charging circuit works. At the moment, that seems like a tall order.  

Since most drivers drive less than 40 miles a day, a typical 240-volt charger can replenish that charge in about four to five hours, and even a standard 120-volt household outlet can recharge a typical car overnight.  A smart system that can make informed decisions on when to charge can help maximize our current electric infrastructure and save the owner money.

–Eric Evarts

Consumer Reports has no relationship with any advertisers or sponsors on this website. Copyright © 2007-2013 Consumers Union of U.S.

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