# RESOURCES FOR TEACHERS: Electricity Uses Math

The fundamental unit of electric energy consumption per unit time is the watt, in which we express the power rating of an electric appliance. For example: your color TV set might use 200 watts; a kitchen can opener, maybe 180 watts; the frost-free refrigerator, perhaps 400 watts; your dishwasher-1,300 watts; and, that deluxe, super hair dryer, the one your daughter dims the room lights with, can use a whopping 2,000 watts.

A kilowatt (kW) is 1000 watts, so that monster hair dryer consumes power at the rate of 2 kW.Power is simply the rate of consumption. Think of your car. If your velocity is 60 mph, this is your rate of travel. Keep that rate of travel up for one hour and you will have traveled 60 miles. Run that dryer continuously for 1 hour and you will have consumed 2 kWh of energy. Energy is power summed over time. Run your TV set for 1 hour and you end up using 0.200 kWh of energy.

Here is a little simple math to help you understand:

Let’s do some math.

Run your TV set for 6 hours a day for an entire month, and your energy bill for that usage will be:

In this example, we assume our cost for electricity is an average value of a dime a kWh, which is not a bad approximation for a residential customer. Certainly with higher electricity costs, higher operating costs result; the same with greater hours of usage.

Now for the super hair dryer-

Let’s say your daughter uses it for 15 minutes every day for one month, so 30 days times 0.25 hours per day is 7.5 hours of usage for the month. Here we go….

As you can see, just because an appliance uses more power, it does not mean it will use more energy, unless you know how long the appliance operates. The TV costs more to operate, yet it is ten times less in power consumption than the super hair dryer; but the TV set is on many hours more than the hair dryer.

i don`t really understand . how do u come to this $0.10

thanks for your kindness.