Your father always told you to turn off the lights when you leave a room (after reminding you that money doesn’t grow on trees). But your best friend told you that turning things off and on wastes more energy than it saves. Who’s right? Here, a guide to when to power down.

Lights: Turn them off whenever you leave a room. “Energy isn’t wasted by turning lights off and back on, and the bulb’s life span isn’t noticeably reduced,” says Bill Prindle, deputy director of the American Council for an Energy-Efficient Economy. A 100-watt bulb left on eight hours a day burns $30 worth of electricity a year.

Car: Running a car emits more pollutants than restarting it would, so if you’re stopped for more than a minute, turn off the engine. And spread the word, says environmental activist Laurie David, who co-produced the documentary An Inconvenient Truth. She started a no-idle program at her children’s school. “Parents waste gas idling in car-pool lanes for 20 minutes,” she says.

Computer: Program it to “sleep” after 30 minutes of nonuse, which can cut power demand by up to 90 percent. Stepping away for longer? Turn it off and also unplug the surge protector, which draws energy even when the computer is off. Booting up again uses the equivalent of only two seconds of run time and won’t hurt the hard drive. Monitors are a big energy drain, so turn them off after 20 minutes of nonuse.