The Green Lantern

You Charged Me All Night Long

Is it a sin to leave your cell phone plugged in overnight?

A cell phone charging

This column was first published in Slate in 2009.

I always charge my phone, laptop, and MP3 player overnight—even though it only takes a few hours to get them fully charged. Should I be losing sleep over this? Would it better for me to charge my electronics during my morning commute, by plugging them into the car charger?

As far as environmental sins go, you can file this one in the venial category. Yes, charging your gadgets for longer than necessary wastes some energy. Will better habits significantly reduce the footprint of your techno-lust? Not likely.

Let’s start with the most ubiquitous mobile device—the cell phone. What happens if you leave yours plugged in all night? According to measurements from Lawrence Berkeley National Laboratory, the average cell phone draws 3.68 watts of power from the outlet while it’s charging and 2.24 watts when charged. Let’s take the worst-case scenario and assume that you’re over-juicing a charged battery for the entire night. Leave the average phone plugged in for eight unnecessary hours, and it’ll use about 0.018 kilowatt-hours of electricity. Do that every night for a week, and the figure rises to 0.13 kWh; every night for a year, and you’re looking at a grand total of 6.5 kWh of electricity.

Given that the average American’s residential electricity consumption is more than 4,000 kWh each year (PDF), the Lantern doesn’t think that a handful of kilowatt-hours are worth much tossing and turning. You could do way more for the planet, for example, by swapping out a single incandescent light bulb in your home for a compact fluorescent one; as the Lantern pointed out in a previous column, that simple action alone can save 126 kWh a year. Plus, charging your gadgets while you sleep has the added benefit of shifting a tiny fraction of your energy usage from the daytime, when demand is highest, to the nighttime, making things just a bit easier on your local grid.

What if you leave your phone charger plugged in all the time, even when the phone itself isn’t attached—how much vampire power would that suck up? Again using the Berkeley Lab figures, if the average charger is plugged in for the entire 8,760 hours of the year, it’ll use about 2.3 kWh of electricity. As Cambridge professor David MacKay notes in his book Sustainable Energy—Without the Hot Air, obsessively unplugging your charger is like “bailing the Titanic with a teaspoon.” By all means, do it, he says, “but please be aware how tiny a gesture it is.” (He goes on to note that, according to his calculations, keeping your phone charger unplugged for a year saves as much energy as skipping a single hot bath.)

Your iPod or Zune probably isn’t worth worrying too much about, either: According to Chris Calwell, the founder of energy efficiency consulting firm Ecos, digital music players only draw about 0.25 to 0.4 watts when fully charged.

What about laptops? If you got yours in the last few years, it may not be much of a nighttime energy hog. According to figures from the University of Pennsylvania’s IT department—which looked at several laptops purchased between 2005 and 2009—today’s laptops draw between one to three watts when switched off but plugged in, and roughly the same amount in sleep mode. That puts them in about the same ballpark as cell phones. A laptop that’s idle, but not asleep, will draw closer to 15 to 20 watts. (Older computers may be less efficient, though a 2003 report showed roughly similar figures for power usage in sleep and on modes [PDF].)

Charging your gadgets during your commute sounds like a good idea, since it would allow you to monitor their progress and unplug them when they’re done topping up. Still, the modest savings you make could easily be undone by the inherent inefficiencies of your car’s electrical system. Transforming gasoline’s chemical energy into electrical energy is a very inefficient process—at every step of the way, from the engine to the alternator, a portion of the gasoline’s original energy content gets lost. By the time you plug your cell phone or iPod into your cigarette lighter adapter, you’re only getting about 10 percent of the energy you initially put into the system. In a hybrid car, that figure might rise to about 20 percent. By contrast, the electricity that comes through your outlets at home delivers about 30 percent of its source fuels’ original energy. All else being equal, then, using a wall socket is going to be more efficient than plugging in to your car.

Is there an environmental quandary that’s been keeping you up at night? Send it to, and check this space every Tuesday.