When you look on most of the appliances in your home, you will see several numbers, including the volts (v), hertz (hz), and the watts (w). Sometimes you may have to look in the manual that came with that appliance to find these numbers. The main number we need to figure out the energy that that appliance uses is the watts, which is a unit of power.

The unit for measuring energy in the metric system is the Joule (J) or a newton meter (N m). The equations for various kinds of energy yield joules, for example:

or as units

In order to determine the energy used by an appliance, you also need to know how long, or the time, that it was turned on. The unit of the watt is a unit of power, or the energy used per time. The equation for power is:

or as units

The unit of a joule of energy is a very small amount of energy and therefore the watt is a very small amount of power. While many appliances are rated in watts, it is common to see power measured in kilowatts (kw) or thousands of watts.

Rating Your Appliance

The electric company actually bills you for the amount of energy that you use. But your appliances are rated by the amount of power that they use. You can easily find the amount of energy used by solving the power equation for energy. You get:

Now you can plug in the power in watts that you found on your appliance and the time in hours. When you do that, the units come out to be kilowatt hours (kw hr). While you could convert the units into joules, it has become customary to leave the energy in the units of kilowatt hours.

Now all you need to know is how much your electric utility or company charges you for energy. This will usually be in the units of $ per kw hr. Then you just multiply the kw hrs by the $ per kw hr, and you end up with the cost for that appliance for the amount of time that you have used it.