source:
http://staff.washington.edu/corey/power.html Measuring Electric Power at Home Without Special Tools
Using just your watch and the electric meter outside your house, you can accurately and safely measure the power consumption of various household appliances and devices. Here's how:
Procedure overview:
First with the device in question turned on, then with it off, you time how long it takes the wheel on your electric meter to make one revolution. Then using the simple equations below, calculate how much power the device is using when on. (This works because the meter records energy use and power is energy per second.) Second, for devices which cycle on and off automatically, determine percentage of on-time on by sampling.
Detailed Steps:
Make sure all other electrical devices in your house are consuming a steady amount of power (or are turned off). You may want to temporarily flip off the circuit breakers to your refrigerator, water heater, furnace, and any other timer or thermostatically controlled appliances which might switch on or off automatically during your measurement.
Turn on the device you want to measure and accurately time how many seconds it takes the wheel on your electric meter to go around once. If the time is less than 20 seconds, you may want to get a more accurate measurement by timing several revolutions and dividing the total time in seconds by the number of revolutions. Call the time you measure T2.
Turn off the device under test and time a revolution as above. Call this time T3.
If you already know how much energy is indicated by one turn of the wheel on your electric meter, skip step 4. The number is often printed inconspicuously on the face of the meter (Kh=3, for example, means 3 watt-hours/revolution). Otherwise...
Determine how much energy is indicated by one turn of the wheel on your electric meter: switch on a high-power device of known wattage (such as a space heater or hair dryer or enough lights bulbs to add up to about 1000-1500 watts). Call this known number of watts Wk. Time a revolution as in steps 2 and 3. Call this time T4.
You can now calculate Kh: the number of Watt-Hours per revolution on your meter:
Kh = ( Wk * T3 * T4 ) / (3600 * (T3 - T4))
Knowing Kh, you can calculate total watts your house was using in steps 2-4 from rotation times:
W2 = ( 3600 * Kh ) / T2
W3 = ( 3600 * Kh ) / T3
The device you wanted to measure in step 2 is consuming (W2 - W3) watts or:
DeviceWatts = (W2 - W3) = Kh * 3600 * (T3 - T2) / (T3 * T2)
I was thinking I'd just call Miss Cleo and ask her to read my meter.