Berkeley Lab

Using an Electromechanical Meter to Estimate Leaking Electricity

All electromechanical kilowatt-hour meters have a rotating disk in them. The disk’s rotational speed is proportional to the amount of electricity consumed. This disk turns even when very small amounts of electricity are being consumed. You can calculate the electricity use by timing the disk revolutions. The calculation requires the “Kh” value, which is the factor used to convert rotations into units of energy called watt-hours (Wh). On most US meters the Kh is 7.2 — so each full rotation of the disk corresponds to 7.2 watt-hours of electricity. Most kWh-meters list the Kh on the front of the meter.

First, find the Kh value on the front of your meter and write it down. Then measure the amount of time it takes for the disk to make one complete turn. Write down the time in seconds. (Remember, there are 60 seconds in a minute, so multiply the number of minutes by 60 to get seconds.) Do this 3 or 4 times to ensure accurate results. Here’s an example of what your measurements might look like.

This meter’s Kh = 7.2

Measurement Time (for one rotation) Power
#1 582 seconds ______ watts
#2 600 seconds ______ watts
#3 609 seconds ______ watts
#4 597 seconds ______ watts

Then calculate the leaking electricity in WATTS for each measurement. The formula is:

WATTS = Kh (watt-hours) x 3600 (seconds/hour)/ Time (seconds)

For example, if your Kh is 7.2 and the disk takes 10 minutes (or 10×60=600 seconds) to go around once, then the total leaking electricity is:

7.2 Wh times 3600 seconds/hour divided by 600 seconds, or:

7.2 x 3600 / 600 = 43.2 watts

If you’d like some practice using this equation, calculate the watts for the numbers in the table above. When you are finished, compare your results to our answers.

Add all the watts and divide by the number of measurements to get an average value for leaking electricity in your home.

Feel free to print this page for future reference. You can use the following table for your measurements.

This meter’s Kh = ______

Measurement Time (for one rotation) Power
#1 ______ seconds ______ watts
#2 ______ seconds ______ watts
#3 ______ seconds ______ watts
#4 ______ seconds ______ watts