In part 1, we went over the very basics of the kWh, but in part 2 we'll dig down a little deeper.
It's hard for many people to understand why you need to multiply Watts (instantaneous value of power) by a unit of time (in our case our unit of time is the hour. The minute or second are other units of time) to get total energy. The technical answer is that one way to represent Energy is by the Joule, just like one way to measure Distance is by the Meter. Taking this a step further, one way to represent Power is the Watt. A Watt is equivalent to a Joule/Second. If you need to get total Joules (which is energy). You need to multiply the Watt (Joule/Second) by total number of Seconds an appliance is in use, which gives you total Joules!
So why is your power bill in kWh? Well, it could just as easily be in Joules, or Wh, or kWs (kilowatt seconds), but kWh is the convention power companies adopted. Just like car companies (in the US at least) adopted miles per hour for the speedometer. They could have just as easily adopted feet per second for your speedometer. So the speed limit on the interstate could be 110 Feet/Second instead of 75 Miles/Hr!
If you have a kill-a-watt, you'd be able to see that your flat panel TV probably consumes somewhere between 213 Watts (LCD) and 339 Watt (plasma). If you want to figure out how much energy your plasma TV will consume if you left it on for 30 minutes you'd have to do a few simple calculations.
The average plasma TV consumes 339 Watts, but we'll uses 340. If the TV is on for 30 minutes, that equals one-half of an hour (.5 hrs.). Therefore, the total amount of energy you have consumed is:
Total Energy (kWh) = (340 Watts)*(.5 hrs) = 170 Wh = .170 kWh
If you live in the South, you probably pay somewhere around 9 cents/kWh. To figure out how much 30 minutes of "The Simpsons" costs you, just multiply the total energy the TV consumed (.170 kWh) by your electric rate ($.09/kWh) to get $ .0153. So about 1.5 cents! I'd say laughter is worth 1.5 cents! The TV is a relatively small consumer of power compared to your Heating/AC, water heating, washer/dryer, or your total lighting energy consumption. Now that you are armed with knowledge about the kWh, look at your big consumers of power, and figure out how to use them less (lowering the time part of the "Total Energy" equation)!