SMART OFFICE TOOLKIT – Energy Cheat Sheet
Smart Office Toolkit – Energy Cheat Sheet
How much electricity do appliances use?
The easiest is to just look at the label! Nearly everything you can plug into the wall has a label that
says how much electricity it uses. It may be printed directly into the plastic or metal and you may
have to hunt for the label. It's often located on the bottom or side of the device, or possibly where
the power cord enters the unit. If the device is powered with an AC/DC adapter, the electrical
rating is usually listed on the adapter itself.
If the label only gives the number of amps and not the number of watts, then just multiply the amps
by the volts to get the number of watts. Most electricity in South Africa is 220/230 Volts,
alternating at 50 cycles per second (50Hz) and Amps x Volts = Watts. So a hot plate that uses
6 amps uses 6 x 230 = 1380 watts. Most countries use 220/240 volts, but America and Japan use
120 volts.
Note that if a device is powered by a transformer (one of those great big plugs), then the
transformer has converted the electricity from AC to DC, so you need to multiply by the DC
voltage, not the AC voltage of 120. For example, if the device says "INPUT 9V, 0.5A", then that’s
9 volts x 0.5 amps = 4.5 watts.
You may have noticed that appliances may be labelled 220, 230, or 240 volts. Appliances are
actually designed to accept a range of voltages, between 220-240 volts, and the exact voltage
coming out of your electrical socket can vary depending on conditions at the power plant and in
your own home. Let's just agree that when we say 240 volts, we understand that it's actually a
range from 220-240 and just use 220 volts for your calculations (unless you're in North America or
Japan, in which case you probably have 120 volts).
Your device might actually list a huge voltage range, like 100-240V. That means that it will work
with any country's voltage. For your calculations, use the voltage for the country where you're
plugging the device in.
A few important notes
The amount of electricity listed on the label is the maximum amount that the appliance will ever
use. For example, a 300-watt refrigerator will only run at 300 watts when the compressor is
running (which is when it makes that humming sound, indicating that it's actually chilling the air
inside). Most of the time the fridge just sits there, using only 5 watts or so for its electronics, but
picks up when you open the door.
If the amount of work done by a device varies, then so does its energy use. (e.g., a stereo that can
be turned up or down, an oven that can be set at various temperatures, a fridge that sometimes
runs and sometimes doesn't, a computer that sometimes spins its various drives and sometimes
