Mr. Green is busy on his world-wide publicity tour for his new book. In the meantime, here's a Mr. Green classic column from June 2007.

Hey Mr. Green,
I usually get 47 to 50 miles per gallon on my 2005 Prius and pay between $2.50 and $2.75 per gallon for gas. Is it cheaper, or better for the environment, to charge gadgets like my cell phone, iPod, and laptop in my car or in my house? I'm in the car about two hours a day. —Joe in San Francisco

I could be lazy and simply tell you to worry less about your toys' relatively small energy use and more about their toxic guts and batteries--and to make sure you safely recycle them. But this is such an intriguing question I couldn't resist poking around for an answer. Though off-the-grid energy production appeals to Americans' do-it-yourself spirit, it turns out that a gasoline engine is not a very efficient device for such efforts.

Using your Prius to charge your cell phone will cause you to emit about 80 percent more carbon dioxide than plugging in at home. (The cost is slightly higher too.) There is, however, one exception: You can get "free" energy if the car's battery is fully charged and the braking system is generating enough power so that the gasoline engine doesn't have to run the alternator to charge the battery. Though this is generally not the case, it might occasionally occur as you roll down the famously twisty Lombard Street or one of your city's other steep hills.