It has to do with the way your meter read inductive loads, and a
thing called power factor.
It's called reactive power and it has to do with the fact that a 50
watt motor will actually draw more current than the 50 watt light
bulb. The extra current draw is a sine wave and will be slightly
lagging the voltage across the motor.
Because the motor is mostly composed of coils of wire. A certain
amount of power is used to set up a magnetic field to make the motor
work. Since this is alternating current... This field builds up and
collapses 60 times a second (the line frequency). While the field is
building up, it is sucking current from your power company. When the
field collapses, the power is added back to the electrical system (and
goes back in the direction of the power company). The net effect is
that it is not metered, or metered at a net of zero, either way, you
don't pay for it). Heavy commercial users of electric motors have
special electric meters that measure reactive power and they DO pay
for it, but not most homes.
Thus, separate from the actual power that is delivered from the motor
(which you do pay for), you have a magnetic field building up and
collapsing 60 times a second which is required to make the motor run.
Even though you don't pay for this out-of-phase current, it gets added
to the current flowing to the wires in your house.
There is the concept of true power (which your electric meter reads)
and apparent power for reactive (transformer, motor, or capacitor
loads). If you have any type of load other than incandescent lights
or electric radiant heating, the reactive power will always be greater
than the true power. The ratio of true power/apparent power is
called the power factor or p.f. If they can't measure, most
utilities will assume that the power factor is 0.80
Thus, a power company serving a residential subdivision with a 0.80
power factor must provide capacity of 1 / 0.80 or about 25% more
current (or reactive power) than they are billing for. Somebody has
to pay for this additional capacity in transmission and distribution
transformers, and increased wire size, even though the power company
can only charge for true power consumed.
This is why transformers are rated at reactive power levels (VA and
KVA) i.e. volts x amps instead of watts.
It's a difficult concept to understand and others can probably explain
better than me, but I took a stab at it.
Beachcomber
