So I heard recently that kids' cell phone chargers are using up the world's energy supplies, and creating a big carbon footprint that will cause global warming. The helpful hints are for you to "stomp out" those carbon footprints, and tell your kids to unplug the chargers when not in use. Or ground your kid for a week.
Tell your kids not to worry, we'll put their fears to rest. And you can un-ground them, because today, You Do The Math!
I just happen to have a year old Motorola that has a 500 milliamp "wall wart", aka transformer, which supplies 5 volts to the cell phone for charging. Before doing any math, we have to realize that 500 milliamps is actually 1/2 amp. What you do is multiply volts by amps to determine watts. So, this should supply about 2.5 watts to the cell phone.
Compare that to your energy saving, 25 watt, spiral, fluorescent light bulb, and you can guess where this is headed.
However, the wall warts have heat losses in the transformer, and use more energy than they supply the phone. When tested at the wall outlet, my meter tells me it uses 28.9 milliamps while charging. Since the wall supplies us with 120 volts, we multiply the two, and get about 3.5 watts.
To calculate energy use, simply multiply power by time. If I charge this cell phone for 10,429 hours at 3.5 watts it will use 36.5 kilowatt-hours of energy. This is the same amount of energy in a gallon of gasoline. At least, that's what you will be billed for. The actual energy used is about 3.7 times that amount. In a previous installment of You Do The Math I used Department of Energy statistics to determine that the US's coal powered utilities as a whole are between 22% and 32% efficient, depending on the type of coal they use. (Interestingly, that's the same range as cars' and trucks' gasoline engines.)
If you take the average of 27%, this means that your utility probably uses 3.7 units of energy for every 1 unit you get out of your wall outlet. Take the 10,428 hours of cell phone charging, divide by 3.7, and we get 2,819 hours. This is how long it takes to use the same amount of energy in a gallon of gas.
Assuming your cell phone is similar, charging it for 7 hours a week (pretend you talk as much as your 13 year old daughter), it will take about 403 weeks, or about 7.75 years to use up a gallon of gas. If your squawk-box-for-a-daughter rides her bicycle 20 miles in the next 8 years, she made up for all her yappin, because she saved a gallon of gas.
Ah, but what about leaving it plugged in 24 hours a day? My charger uses 0.45 milli-amps when the phone is not attached. Multiply by 120 volts, and that's 0.054 watts. Divide 36.5 kilowatt-hours by 0.054 watts, divide by 3.7, and we get 182,683 hours.
What did you say? Yes, 182 THOUSAND hours. So, your daughter can leave a cell phone like this one plugged in for 7,612 days, or about 21 years, to equal a gallon of gas. By that time, you'll be worried about which junior high your daughter is planning to send your grand kids.
Some of you are asking, "What about my husband's huge cell phone that is essentially a hand held computer? Don't those use a lot more power?" I'm sure they do, since they play music, have lots of processing power, lots of memory, lots of internet surfing, and lots of video downloads. So, I tested a first generation iPhone that does all that, which went on the market in 2008.
When the wall charger was left in the wall, it used about 1.4 milliamps at 120 volts. That comes to 0.168 watts. So, it burns a gallon of gas' worth of energy in 58,719 hours, or about 6.7 years.
When actually charging, the iPhone averaged about 16.7 milliamps, a tic over 2 watts, which is actually less power than my little cell phone. For the sake of argument, if you charged the iPhone 7 hours a week, it would use a gallon of gas's worth of energy in 703 weeks, or about 13.5 years.
Seems like an energy hog. But, bear in mind the iPhone does what your desktop PC does. Yet, a modern, fast desktop burns energy 100 times faster than when an iPhone is actually charging. At that rate it uses a gallon of gas's worth of energy in 49.3 hours, or about 2.75 weeks if you use it 18 hours a week. Yet, even a coal-guzzling, desktop PC saves energy if it can spare you a drive in the car once every 2.75 weeks.
Compare the PC to the clothes dryer, which burns energy at a whopping 5000 watts. It uses a gallon of gas's energy in a mere 2 hours. So, if you use a clothes line to dry 2 loads of wash, you could leave your cell phone charger plugged in for 21 years. I think you better remove the green "Energy Star Compliant" sticker from your dryer.
Consider a 15 mpg van or large pickup truck that uses a gallon of gas in half an hour while city driving, as opposed to a 22 mpg car that would make that gallon last 45 minutes. At that rate, if you drove a car a couple days out of the year, and saved a gallon of gas, you could leave your cell phone charger plugged in 21 years.
If you ride a 50 mpg moped or motorcycle 50 miles, perhaps one day's worth of commuting, you can use about 2.3 gallons of gas less than the light truck, and that will allow you to leave your cell phone charger plugged in for 48 years.
I hope you get my point. To be environmentally conscious, don't sweat the small stuff, especially cell phones.