The Green Lantern

Keep On Plugging

Should you run your laptop off battery power or use a charger?

This column was first published in Slate in 2008.

Illustration by Robert Neubecker.

Illustration by Robert Neubecker.

I’m a committed laptop user, but most of the time, I’m just using it at my desk. Do I save more energy by keeping it plugged in, or should I use my charger only when my battery gets low?

First, some good news: Despite your desk-bound ways, you’re using a laptop, which is already a greener choice than a desktop. It’s not just that you don’t have a monitor to plug in—although that’s a big part of it. Laptops are typically designed to be more energy-efficient than their desktop counterparts, and all in all, they are estimated to use as much as 80 percent less energy while operating. There’s a simple reason why laptops are more efficient: They need to be. Waste energy on a desktop, and you’ll get a slightly higher electricity bill. But if your laptop isn’t energy-efficient, that means either you’ll need a bigger battery (and a heavier machine) or you’ll spend half your time hunting for an outlet at your local coffee shop. (Laptops likely require less energy to manufacture, too—although regardless of which machine you use, one of the greenest things you can do is hold off on buying a new computer.)

Getting back to your question, start with the assumption that your computer will operate exactly the same whether or not it’s plugged into the wall. In that case, you’re probably better off staying plugged in, because energy is lost in the process of charging the battery, storing the electricity, and then powering the computer from the battery. A report (PDF) prepared by the Natural Resources Defense Council five years ago estimated that running a laptop from AC power is about 20 percent more energy-efficient than doing it off a battery. Even if battery charging systems have improved since then, common sense suggests that using AC power requires less energy. (Some laptop users contend that keeping a laptop plugged in damages the life of the battery. If so, this would be a tougher call, since batteries require an awful lot of energy to manufacture, and there’s an environmental cost to recycling a spent one. Different manufacturers give slightly different answers: Lenovo and Dell told the Lantern your battery should be fine if your computer stays plugged in; HP says you should remove the battery if you are running on AC power for weeks at a time; and Apple suggests you should unplug and run off the battery every once in a while. Check your manual, but the Lantern thinks you should be OK using AC power most of the time.)

Still, there’s a catch—although it’s one you can do something about. Most laptops are set up to use less energy when they aren’t plugged in, since battery life is at a premium. As soon as they start receiving AC power, however, they’re often set to start running at higher speeds—and thus use more energy. If you’ve never touched your laptop’s power settings before, chances are it uses more energy when it’s plugged into the wall.

It doesn’t have to. Your laptop should make it easy to change your energy settings so that they’re the same whether or not your laptop is plugged in. When you do that, your computer will probably run a little slower, your screen may be a little dimmer, and your machine may go to sleep a little faster when you walk away from your desk. But unless you spend most of your time playing Left 4 Dead, you probably won’t mind those changes. (A quick aside: For those wondering whether it makes sense to turn off their computer or leave it on idle, it usually makes sense to shut your machine down if you are going to be away for any extended period of time.)

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