Hey Mr. Green,

I sometimes make a point to plug my phone into the car charger when I drive, thinking that the energy required to charge the phone or iPad doesn't decrease fuel efficiency and therefore is kind of free. Is that true?

—Richard, in San Francisco

Alas, there is no free energy. The electricity produced when your engine spins an alternator requires a teeny weeny bit more gasoline to create additional electricity for charging. The car's voltage regulator allows the alternator to generate more or less power, according to demand, and your phone adds to that demand. So you'll use a teensy-weensy bit more net energy overall than you would by plugging the phone in at home, because power plant dynamos are typically 30 percent to 40 percent efficient, while gasoline engines are from 14 percent to 30 percent efficient, depending on speed and other variables. Moreover, if your power company does not rely heavily on fossil fuel, you will be putting a tad less carbon dioxide into the air if you charge at home instead of in the car.

Note that we’re talking a minute amount of energy here. More important is to unplug the charger as soon as a phone or other device is fully charged. This is because many chargers continue to suck electricity after the battery is fully charged, which is why this wasted electricity is called "vampire power." You don't need to worry about unplugging if you have a "smart" charger, because these automatically stop drawing power when the battery is fully charged.--Bob Schildgen