If you look carefully at a stereo, hair dryer, or other household appliance, you find that most devices list a “power rating” that tells how many watts the appliance uses. In this section you will learn what these power ratings mean, and how to figure out the electricity costs of using various appliances.
The three electrical quantities
We have now learned three important electrical quantities:
Paying for electricity
Electric bills sent out by utility companies don’t charge by the volt, the amp, or the ohm. You may have noticed that electrical appliances in your home usually include another unit – the watt. Most appliances have a label that lists the number of watts or kilowatts. You may have purchased 60-watt light bulbs, or a 900-watt hair dryer, or a 1500-watt toaster oven. Electric companies charge for the energy you use, which depends on how many watts each appliance consumes in a given month.
A watt is a unit of power
The watt is a unit of power. Power, in the scientific sense, has a precise meaning. Power is the rate at which energy is flowing. Energy is measured in joules. Power is measured in joules per second. One joule per second is equal to one watt. A 100-watt light bulb uses 100 joules of energy every second. Where does the electrical power go?
Electrical power can be easily transformed into many different forms. An electric
motor takes electrical power and makes mechanical power. A light bulb turns electrical power into light and a toaster oven turns the power into heat. The same unit (watts) applies to all forms of energy flow, including light, motion, electrical, thermal, or many others.
Power in a circuit can be measured using the tools we already have. Remember
that one watt equals an energy flow of one joule per second.
Amps = flow of 1 coulomb of charge per second
Volts = an energy of 1 joule of energy / coulomb of charge
If these two quantities are multiplied together, you will find that the units of
coulombs cancel out, leaving the equation we want for power.
Watts equal joules/second, so we can calculate electrical power in a circuit by
multiplying voltage times current.
P = VI
power measured in watts; voltage in volts; current in amps
A larger unit of power is sometimes needed.
A 1500-watt toaster oven may be labeled 1.5 kW.
kilowatt (kW) is equal to 1000 watts, or 1000 joules per second.
Horsepower – another common unit of power often seen on electric motors
1 horsepower = 746 watts.
Electric motors you find around the house range in
size from 1/25th of a horsepower (30 watts) for a small electric fan to 2 horsepower (1492 watts) for an electric saw.