In the September issue of ELECTRICAL CONTRACTOR, there was an
Industry Watch story titled “Efficiency from the Wall to the
PC,” about an organization that is campaigning for more
efficient power “cords.” There was a reference to a quote
indicating only 50 percent of the power that leaves the outlet
reaches the PC because energy “leaks” out of inefficient power
cords. However, a typical PC line cord has 0.3 ohms of
resistance. If the computer consumes 720W, that is 6A on 120V
circuit. The power loss in the cord is 6A^2*0.3 ohm = 7.8W.
Relative to the computer itself, that is approximately a 1
percent loss of power delivered from the outlet, a 99 percent
efficiency rating. A few readers wrote to us about the
inaccurate terminology. It should have read power “supplies,”
and, of course, energy does not literally “leak” out of the
power cord. It is lost within the inefficient components, such
as the computer’s power supply. As with most power quality
issues, Ohm’s and Kirchoff’s Laws still apply here. These rules