Whenever I disconnect my mobile phone from its charger, it tells to me to unplug the charger “to save energy”. I obey. But how much energy am I saving? Very very little. Handily, David Mackay has actually measured the power consumption of a mobile phone charger with no phone attached. It is 0.5 W. W = Watt, the standard unit of power, it is 1 Joule (J) per second, where Joule is the standard unit of energy. Thus if you leave a charger plugged in for a day without a mobile attached, you use 24 times 60 time 60 times 0.5 = 43,200 J.

This may sound a lot, but let’s try and put it in context. I will probably be flying to Boston in the summer. How much energy will this consume? Handily, David Mackay has also estimated the energy consumed in a typical long-haul flight. It is about 50,000,000,000 J, which is a lot more. In fact it is about a million times more, which means to consume the same amount of energy as my flight to the States, I would have to leave the charger plugged in for a million days, which would take until the year 2284.

This is a long time. To be honest I want a new smartphone so I will probably get one soon, long before 2284, when I will be over 300 years old.

So I guess my conclusion is that you can do small things to consume less energy but if you want to make a difference, don’t worry about the charger, worry about the big things, like flying. If you want to know what these are David Mackay’s website is a great place to start.