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The electrical power used in operate an
electrical device is defined as the potential energy or
voltage times the current passing through the device.
This could also apply to a whole electrical system, such
as the the power used in running your household
appliances. This is compared to the mechanical
definition of power as the work done over a period of
time. The electric company uses the power used over a
period of time to calculate the energy used and thus
your electric bill.
It is compared with the power required
to do some work over a period of time.
Questions you may have are:
-
How do you determine electrical
power?
-
How do we measure electrical power?
-
How is your electric bill
calculated?
Determining electric power
The electrical power required to operate
a device is the input voltage times the current
required.
P = V * I
where P = power, V = voltage and I =
current in amperes.
Electrical power is measured in watts.
If the amount of watts is large, kilowatts are used. 1
kilowatt = 1000 watts, just like 1 kilometer = 1000
meters. The abbreviation for kilowatt is usually kW.
If you look at the top of a light bulb,
you will see its power rating. One example is a 100 watt
light bulb. You can use the equation for electrical
power to determine the amount of current passing through
that light bulb.
If your house voltage is 110 volts, then
you can see that:
100W = 110V * I
Thus I = 100 / 110 = 0.91 amps.
You can also find the resistance of the
light bulb, using Ohm's Law V = I * R.
110V = 0.91A * R
Thus R = 110 / 0.91 = 120.9 ohms.
Comparing with mechanical power
The standard or mechanical definition of
power is the work per unit time. (See Work for more on
that subject.) In other words, power equals work divided
by time.
P = W / T
where P = power in watts, W = the work
done in joules and T = the time of measurement. Since
energy is often defined as the ability to do work, let's
substitute energy E for work and rearrange the equation:
E = P * T
Thus, the electrical energy used is the
electrical power times the time. If we measure the
electrical power as kilowatts and the time as hours, we
get the energy used by an electrical system in terms of
kilowatt-hours. That is the unit of measurement the
electric company uses when determining your bill.
Knowing about electrical power can help
you in understanding how your electric bill is
calculated. The electric company sends you a bill
determined by the amount of work the electricity has
done or amount of energy expended in kilowatt-hours.
Most homes have an electric meter outside that measures
the amount of electrical energy used by the house over a
period of time.
Many electric companies charges about
$0.07 per kilowatt-hour. Thus, you multiply the number
of kilowatts of electricity you use times the amount of
time you use it and multiply that by $0.07 to get your
electric bill.
For example, if you used a 1500-watt
hair dryer for 100 hours in a month at a cost of $0.07
per kilowatt-hour, the electric company would bill you
for:
1500 watt * 100 hours = 150,000
watt-hours = 150 kilowatt-hours.
Thus your bill would amount to:
150 kilowatt-hours * $0.07 / kW-hr =
$10.50.
Electric power is voltage times current.
Your electric bill is based on the electric power times
the time used, in kilowatt-hours. Knowing how much power
you used and the electric rate charged, you can
determine your electric bill. |
