Here’s a riddle for you:
If the Chevrolet Volt and other plug-in hybrids can go 40 miles on a full battery charge, and the federal government says 78 percent of Americans travel less than that a day, why are GM, Ford, and other automakers frantically working with cities and utility companies to develop public charging stations where electric cars might park and charge during the day?

It’s expected to take over eight hours to charge a Volt on a 110-volt outlet. Couldn’t owners just charge them up at night in their garages at home? It would be convenient and inexpensive. In fact, it’s expected to cost about $1.60 to fully charge the Volt at national average residential electric rates.

According to the Electric Power Research Institute, the United States has enough extra electric capacity to charge one million electric cars at night (although potentially very few at times of peak electrical demand on hot summer days). To provide maximum advantage in energy and cost efficiency, it is commonly held that electric cars should be charged at night.

And plug-in hybrids have a gas engine to get them home even when the battery is flat. So why all the effort to create places for people to charge up when they might not need to, anyway?

As with most automotive inefficiencies, the answer, of course, is human nature. One of the lessons GM learned from its EV1 electric car in the 1990s, and likewise Toyota more recently from its electric-powered RAV4s, is that when people have cars with a limited range, they take every opportunity to plug in and extend that range—even if they’re only stopped at the grocery store for five minutes.

Given that Ford and Toyota also say they are working on pure battery-electric vehicles, these “opportunity charging” stations will likely be necessary to build a market for the cars.

But when it comes to plug-in hybrids that already have a gas engine to take them wherever drivers want, I don’t buy it when you read that they need public charging stations to make them work.

Read our report on converting our Toyota Prius to a plug-in and learn how hybrids work.

—Eric Evarts

Learn about driving green in the Consumer Reports special fuel economy section