How much energy do your smartphone, laptop, and tablet chargers really use? Should you unplug them when you aren’t using them to save power and money? We measured exactly how much power a variety of common chargers use, and how much keeping them plugged in will cost your each year.

You’ve probably heard of “vampire power”—the amount of power a device uses in standby mode when you aren’t using it. But just how much vampire power does a charger use, and is it worth the hassle of unplugging them when you aren’t using them?

How We Measured It—and How You Can, Too

We used a Kill A Watt electricity usage meter to measure the power usage of a variety of popular chargers. They’re currently under $25 on Amazon, giving you an easy way to measure your devices, too. Plug the meter into an electrical socket, and then plug another device into the meter. The meter sits between the two and tells you how much energy the device is using. This is very useful if you want to measure your energy use, allowing you to identify power-hungry appliances and devices that should be replaced or adjusted. Look up the rate your electricity company charges you and you’ll be able to figure out exactly how much that electricity will cost you, too.

So, with a meter in hand and a variety of chargers lying around, we got to work and tested them so you wouldn’t have to.

How Much Vampire Power Does a Charger Use?

We plugged in a variety of chargers—iPhone, iPad, MacBook, Android phone and tablet, Windows laptop, Chromebook, and even Nintendo’s 3DS charger. It was immediately obvious there was a problem with the very idea of our test. Having heard about the evils of vampire power and the need to unplug devices when we’re not using them, we were surprised to see that not a single charger used a detectable amount of vampire power when it was plugged into an outlet.

In other words, the meter’s display read a big 0.0 watts, no matter what charger we plugged into it.

But Surely They’re Drawing Some Power!

It’s not entirely accurate to say that each charger was using 0 watts, of course. Each charger is using some fraction of a watt. And it should certainly be detectable at some point!

With that in mind, we had a new idea—plug a power strip into the meter, and then plug multiple chargers into the power strip. Then, we could see just how many chargers it takes for the meter to be able to measure some noticeable electrical draw.

The power strip itself—despite its red LED light—registered 0.0 watts when we plugged it in. We started plugging in chargers and watched the meter continue reading 0.0, even after several chargers were plugged in.