Partial truths

As a retired electrical engineer, I find it hard to accept claims by people appearing on television news telling us that we are wasting a lot of electricity when we leave our cell phone chargers plugged in all the time. 

The claim is that these chargers still use electrical energy even when not in use and that cumulatively, these chargers use enough electricity to require more thermal electric generators on line.  If we only unplug these chargers, we can shut down some coal burning generators and help our environment.


Most technical people know that unused or wasted energy most often show up as heat. Cell phone chargers are typically cool when not charging even if plugged in and get slightly warm when charging.

One recent television news show with that claim finally motivated me to do some calculations.

The result is that the claims are true, but only partially true.

Typical cell phones use about 2-5 watts when charging and .1 to .2 watts when not charging.  This agrees with my intuition that not much energy is used when plugged in.  Almost all chargers are "switching power supplies" meaning that they turn on to let power through when there is something to charge.  When there is nothing to charge, there is a little bit of power used just to keep the circuits activated and transformers magnetized.

Assuming one cell phone each for the 30 million Canadian population, if we all plug our cell phones in at the same time, it will demand 60 Megawatts from a generator.  This is small in terms of utility power plants but large for independent power producers.

However, if we take the 300 million US population, then the total demand is 600 Megawatts.  Now we can point to that figure and say "see, that is the size of a decent coal burning thermal power plant that can be eliminated if we don't charge cell phones".

But we don't all charge our cell phones at the same time.  So if we use the power usage with the charger plugged in but not used, it is one tenth of this figure.

Now a much smaller power plant of 60MW capacity for the whole US population.

Perhaps we have an iPad, iPod, and a few other chargers.  So triple the figure and we have 180MW.

Still seemingly a big number and still the truth.

However, what is not mentioned up to now is that there are 6,997 power plants in the US with a capacity larger than 1 MW.  The total generation capacity is around 600,000MW.

So the cell phone chargers demand around 0.1% of the generation capacity when charging and around 0.01% of capacity when plugged in but unused.

It cost around 50 cents a year to run a cell phone charger per year.  http://energyusecalculator.com/electricity_cellphone.htm

Note that it is true that if we all unplug our chargers and stop charging our cell phones, that we can indeed shut down a generation plant or two in the US.

It is not that the claim is not true, but that it paints a picture that is not true.

Someone compared it to bailing the sinking Titanic with a teaspoon.  It is true that it helps but it is not the right use of our attention and energy.


Most of us do not have a grasp of the number of generation plants there are and the overall size of the system so we are impressed with large numbers resulting from multiplying with large populations.

Shutting down a power generation plant is a vivid image.  So long as we don't see it as one among several thousand.

Our book club is currently reading Joseph Heath's "Enlightenment 2.0".  It highlights how we need rational thinking and analysis to resist our intuitive feel.

Here is an example of that.

In our modern interconnected life where division of labor narrows us to our own specialties, we depend on experts to for advice.  Unfortunately, these experts often have their own agendas to promote and it is not always easy to judge the validity of their expertise. To make matters worse, mass media is no better at selecting experts than we do and the wrong people get media exposure all the time.

So we have partial truths and it is up to us to decide what else we should know that we have not been told.

How are we suppose to know what we don't know?