Determining kWh Use - Meter-Disk Revolutions

You can also count the meter-disk revolutions on the kilowatt-hour meter to determine the watt-hour consumption of an appliance, just as you can count meter revolutions to determine the kilowatt consumption of an appliance. This is a method of making a quick, accurate check of equipment already in use.

Each meter has a flat aluminum disk with a black mark along its edge. This disk turns when energy is being used. Meters also have a meter constant. The constant is shown on the meter nameplate. A constant "Kh = 7.2" means that for each revolution of the disk, 7.2 Watt-hours has been used (constants will vary with different meters).

To determine how much electrical energy is used by counting meter-disk revolutions, proceed as follows:

1. Find the circuit that supplies the appliance you wish to check, for example, a portable heater. Plug the portable heater into the outlet where the appliance is to be checked, then remove the fuses at the fuse box or trip the circuit breakers until the heater goes off. Replace the fuse or return the circuit breaker to the "on" position. This fuse or circuit breaker serves the circuit you want to check. Remove all other fuses, or trip all other circuit breakers, so that all other circuits are off.

2. Disconnect any other equipment on the same circuit with the appliance to be checked, or make sure that switches are in the "off" position so that all other appliances which are on the same circuit have been disconnected.

3. With a watch, check the number of meter-disk revolutions over a timed period. You may use any period of time. Six minutes makes a good period for equipment that operates continuously. Six minutes is one-tenth of an hour and can be easily calculated. Equipment, such as a refrigerator, that is off part of the time should be checked through at least one "on-and-off" cycle.

4. Determine watt-hours of energy used in the timed period by multiplying the meter constant by the disk revolutions for that period. We will assume the meter constant to be 7.2 (Kh = 7.2).

5. Divide the minutes in the timed period into 60 minutes to determine how many such periods there are in an hour. The six-minute period used will equal 10 periods in an hour. If the check period is 15 minutes, there will be four periods in an hour.

6. Multiply the number of watt-hours you measured in step four by the number of time periods per hour. This will give the total number of watt-hours of energy used per hour.

7. Determine the number of hours equipment is used per month. For equipment that is connected to the circuit continuously, you figure the total hours per month (24 hrs. x 30 days = 720 hrs. per month). For equipment that is used only part of the time, such as a heater, toaster or iron, estimate the number of hours used per month. For this problem, assume that the heater operates for an average of two hours each day.

8. Multiply hours of operation per month by watt-hours used per hour to determine the number of watt-hours used per month.

9. Divide watt-hours used per month by 1,000 to find number of kilowatt-hours used per month. Then multiply the number of kilowatt-hours by the per kWh cost charged by the utility to determine to cost to operate that appliance for that length of time.