Have you ever wondered how much power your house or apartment is using?  Most watt-hour meters are like the one shown below.  Some newer ones have a digital display but there millions of these older models still in use.

Note the Kh 7.2 number on the tag in the photo above.  This says that the wheel takes 3600/72 or 50 seconds to make a complete rotation, when the house is drawing one kilowatt of power.  Using a stopwatch, you can measure how much power your house is drawing from the utility grid.  First, measure the time (T) it takes for the wheel to make one rotation.  Take that number and insert it into the equation below.

Power = 3600/72 X 1/T

As an example, suppose you measured 7.5 seconds for the wheel to make one rotation. Then, using the equation, you would be drawing 6.7 kilowatts of power.

Tapping into the guts of a watt-hour meter to remotely record the energy use is dangerous and illegal.  But, there is a way to monitor power consumption without making any electrical contact with the meter or any other electrical wiring.  Note the black band on the wheel.  I think it would be possible to use a visible red LED and a PIN photo diode to detect this back band and measure the time for each wheel rotation.  Using an RF transmitter, that information could then be sent to a matching RF receiver mounted somewhere inside the house.  A microprocessor or even a computer could then keep track of the time for each rotation and the number of rotations.  The processor could use the information to display the kilowatts of power being used and the total kilowatt-hours of energy consumed for any given time. 

This might make a nice hobby project.