Every Little Bit Helps, But Each One Only Helps A Little – Computational Example

One of the common phrases that is repeated about our need to change our energy consumption habits is that “every little bit helps”. On the top of the list at about.com’s page titled Energy Saving: 13 Tips to Turn Your Cell Phone Green is advice to unplug phone chargers when not in use. That same advice shows up on the US government’s Energy Star page. That advice is okay on the surface, but unfortunately you can find people who believe that it can save a whole bunch of energy if everyone would just follow it.

I pulled out one of my many accumulated phone chargers and read the sticker on the outside. It told me that the input current was 0.2 Amps at a normal household voltage of 110-120 volts. That works out to approximately 24 VA – roughly 24 Watts (ignoring some reactive current). The output of this particular device is a DC current of 0.9 A with a maximum output voltage of 4.8 V. That is just 4.3 W output for 24 W input; not a terribly efficient conversion device.

If I were to leave that particular charger plugged in when not using it for charging my phone, it would stay hot and consume approximately 0.48 kilowatt hours every day, assuming that it draws just as much current when there is a phone being charged as when there is not one being charged. (My math assumes 20 hours per day of wasted power; I average about 4 hours of charging per day to keep my phone operational.)

However, I would be hard pressed to find any savings from making this effort. Perhaps I could make a different choice that would have more effect.

Let’s say that I have been invited to attend a friend’s wedding over a weekend. That friend lives 500 miles away, so it is a reasonably easy drive for an important life event. If energy conservation is terribly important to me and I have determined that is the way I am going to save the world, I could skip the trip. I happen to drive a very efficient car that gets 50 miles per gallon, so I might be tempted to just go ahead and make the drive. My choice to skip the trip could be a lot easier if I only own a gas guzzler. (That is Jevons Paradox at work, just in case you do not understand how maximizing device efficiency does not necessarily lead to overall energy savings.)

Even in my fuel efficient car, I would burn up 20 gallons of gasoline traveling 500 miles each way without any side trips. Each gallon of gasoline contains approximately 40 kilowatt-hours of energy, so avoiding the trip would reduce my annual energy consumption by 800 kilowatt-hours and save me about $55 at current gas prices (plus some wear and tear on my car). Let’s compare that to the daily small sacrifice of unplugging a phone charger.

Assume that the electricity supplying the phone charger was all supplied by burning coal at an efficiency of 33%. Saving 0.48 kilowatt-hours per day would save 5.1 mega joules per day worth of coal. If the power plant is burning bituminous coal with an energy density of 24 MJ/kg, it takes about 210 grams of coal to provide 5.1 mega joules of heat energy. At a representative sample of $60 per metric ton including transportation and processing, the required coal would cost just 1.2 cents per day. It would take 555 days (a year and a half) of unplugging a 24 watt charger to save the same energy value of fuel as avoiding just one 500 mile – each way – weekend trip in a very fuel efficient automobile.

On a cost basis, the fuel savings from unplugging the 24 watt device is just $6.60 if you stick with it for the full 555 days it would take to equal the avoided automobile trip. If I happen to be lucky enough to get my power from a nuclear plant, I would only save the power company about $1.25 worth of fuel by reducing my consumption by 266 kilowatt-hours.

My bottom line – Americans CAN reduce their energy consumption by a substantial amount, but they are not going to do it by unplugging phone chargers. The choices needed to make big reductions come at a higher cost and may not be easy to swallow, especially if the choice is made by someone else.