Examine the electric meter in your house. It is probably in the basement or on the outside of the house. You will see that, in addition to the clocklike dials in the meter, there is a circular aluminum disk that spins between the poles of magnets when electric current goes into the house. The more electric current, the faster the disk turns. The speed of the disk is directly proportional to the number of watts used; for example, it spins 5 times as fast for 500 W as for 100 W.
You can use the meter to determine how many watts an electrical device uses. First, make sure that all electrical devices in your home are disconnected (it is okay to ignore electric clocks and other 2-W devices, which will hardly be noticeable). The disk will be practically stationary. Then connect a 100-W bulb and note how many seconds it takes for the disk to make five complete revolutions.
The black spot painted on the edge of the disk makes this easy. Disconnect the 100-W bulb and plug in a device of unknown wattage. Again, count the seconds for five revolutions.
If it takes the same time, it's a 100-W device; if it takes twice the time, it's a 50-W device; half the time, a 200-W device; and so forth. In this way you can estimate the power consumption of devices fairly accurately.